Are you battling on the PSS, or in Battle Spot? Remember that you can only Mega Evolve one Pokemon per battle, as having 2 Megas in 6-on-6 battles is very pointless, really. In Battle Spot formats, this is a very viable strategy, as you can pick the best Mega to use depending on the matchup.
Even though Hydreigon resides in an awkward Speed tier, Quick Claw is not a good way to rectify it. That's because the item falls under the category of luck/hax because it doesn't activate all the time, and luck-based items are inconsistent to use. Specially-based Hydreigon is also preferred over physical ones because Hydreigon has better Special Attack than Attack, though these sets sometimes incorporate Superpower to hit Blissey (and Chansey) hard. Body Slam is redundant with Hydreigon's Dragon STAB. Here are much better sets to fall back on:
-Dark Pulse
-Draco Meteor
-Flash Cannon
-Superpower/Earth Power
Nature: Mild/Modest
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
or
-Dark Pulse
-Draco Meteor
-Earth Power
-U-turn
Nature: Modest/Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Scarf
While Latias can function in its Mega form, Metagross does it much better, and it needs its Mega Evolution to be effective, as explained below later. I don't think Latias is worth the Mega slot in this team. Heatran, Ferrothorn, and Clefable are reliable Stealth Rock setters you can consider. Stealth Rock is a staple in competitive play because it's an entry hazard that hits all types, and the damage they take
depends on how strong/weak they are to the actual Rock type:
Heatran:
-Stealth Rock
-Lava Plume
-Toxic
-Taunt/Protect/Roar
Nature: Calm
EVs: 248 HP/192 SDef/68 Spe
Item: Leftovers
or
-Fire Blast/Magma Storm
-Flash Cannon/Hidden Power (Ice)/Stone Edge
-Earth Power
-Stealth Rock
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Air Balloon
Ferrothorn:
-Stealth Rock
-Leech Seed
-Power Whip/Protect
-Gyro Ball/Thunder Wave
Nature: Relaxed
EVs: 252 HP/88 Def/168 SDef
Item: Leftovers
Ability: Iron Barbs
Clefable:
-Stealth Rock
-Moonblast
-Thunder Wave/Fire Blast
-Soft-Boiled/Moonlight
Nature: Calm
EVs: 252 HP/172 Def/84 SDef
Item: Leftovers
Ability: Magic Guard
As I already mentioned, the Mega slot should be given to Metagross. Regular Metagross isn't worth it, really, as its Speed is lower than its Mega form's. Mega Metagross, on the other hand, has improved bulk and especially Speed, as already mentioned, and the Tough Claws ability to take advantage of most of its wide move pool. For some reason, it benefits Grass Knot, despite it being a special attack:
-Meteor Mash
-Zen Headbutt
-Hammer Arm/Earthquake
-Ice Punch/Bullet Punch/Grass Knot/Pursuit
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Metagrossite
Ability: Clear Body
or
-Agility
-Meteor Mash
-Zen Headbutt
-Hammer Arm/Earthquake
Nature: Adamant
EVs: 80 HP/252 Atk/176 Spe
Item: Metagrossite
Ability: Clear Body
Garchomp does have a decent move pool, sure, but some of its options aren't that good, like Brick Break, which has redundant coverage with Earthquake. Even though Iron Head (and Poison Jab by extension) let it hit Fairies super effectively, they're not worth using, as non-Togekiss Fairies are hit hard by Earthquake already. Look at the difference below:
252 Atk Garchomp Earthquake vs. 244 HP / 0 Def Sylveon: 232-274 (59.1 - 69.8%) -- guaranteed 2HKO
252 Atk Garchomp Iron Head vs. 244 HP / 0 Def Sylveon: 248-294 (63.2 - 75%) -- guaranteed 2HKO
There isn't much of a difference here, really.
Anyway, on to better sets that provide better coverage and utility for Garchomp; heck, you can have it be the Stealth Rocker on the team if you don't want the above options I suggested:
-Stealth Rock
-Dragon Tail
-Earthquake/Toxic
-Fire Blast/Toxic
Nature: Impish
EVs: 252 HP/164 Def/76 SDef/16 Spe
Item: Rocky Helmet
Ability: Rough Skin
or
-Stealth Rock
-Outrage/Dragon Claw
-Earthquake
-Fire Blast/Swords Dance
Nature: Naive/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Lum Berry/Focus Sash
or
-Swords Dance
-Dragon Claw/Outrage
-Earthquake
-Substitute/Stone Edge
Nature: Jolly
EVs: 12 HP/244 Atk/252 Spe
Item: Salac Berry/Leftovers/Lum Berry
or
-Draco Meteor
-Earthquake
-Fire Blast
-Stone Edge/Stealth Rock
Nature: Naive
EVs: 4 Atk/252 SAtk/252 Spe
Item: Life Orb
or
-Outrage
-Earthquake
-Stone Edge/Fire Blast
-Dragon Claw/Fire Blast
Nature: Jolly/Naive
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Flying moves aren't worth using on Charizard X, as it Mega Evolves ASAP. Fly is one of those cases, as it's rather predictable, and the opponent can switch something resistant to Flying in to take the hit. Charizard X should also use physical moves only, as not doing so will squander the point of having Tough Claws in the first place. Your current set is also walled badly by Heatran, a rather prominent threat:
-Dragon Dance
-Flare Blitz
-Dragon Claw
-Roost/Earthquake
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Charizardite X
Ability: Blaze
or
-Flare Blitz
-Dragon Claw/Earthquake/Dragon Tail
-Will-o-Wisp
-Roost
Nature: Adamant
EVs: 248 HP/24 Atk/180 SDef/56 Spe
Item: Charizardite X
Ability: Blaze
or
-Swords Dance
-Flare Blitz
-Outrage
-Tailwind/Roost
Nature: Adamant
EVs: 40 HP/252 Atk/216 Spe
Item: Charizardite X
Ability: Blaze
Note: for Swords Dance + Tailwind, you don't try to get both boosts at once; you pick one depending on the team you're up against. Swords Dance is for trying to break through defensive teams, while Tailwind for outspeeding offensive ones.
As for Suicune, as weird as this sounds, but the below set is more effective than your above. Mist is situational, as stat drops happen randomly. Mirror Coat also isn't worth it, as Suicune lacks reliable recovery. The below set has served Suicune well since Gen III, with a Gen V twist, and I'll explain why:
-Calm Mind
-Scald
-Rest
-Sleep Talk
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers
As you can see here, Calm Mind boosts Suicune's Special stats, powering up Scald and making you harder to break with (non-critical hit) Special attacks. A Bold nature is used here, since you'll be boosting your Special Defense with Calm Mind anyway, and using a specially defensive investment here may seem tempting, but it's not worth it, as Suicune will be more vulnerable to physical attackers. If Suicune happens to get low on health and/or is crippled by burn/poison/paralysis, use Rest. While this normally leaves Suicune a sitting duck for 2 turns, this is where Sleep Talk comes in. Sleep Talk will randomly pick between Suicune's other 3 moves while it sleeps. So with that, you can either boost up more with Calm Mind, attack with Scald, or do nothing with Rest, as it fails while you're asleep. All these have a 1/3 chance of happening. Sure, Electric and Grass moves are more problematic now, but at least you have teammates to handle those; that, and after some Calm Mind boosts, super effective special Electric/Gras moves won't matter, unless those Pokemon boosted alongside you.
Now, for a brief history (that I can recall) for CroCune (the name that the above set is widely known as). A user named Cromat created this set with Suicune in Gen III. At the time, all Grass and Electric moves were Special, so Suicune could easily set up in the faces of weak/slower Electrics and Grasses to sponge their hits, then use RestTalk to maintain the advantage over them. In Gen IV, physical Grass and Electric moves were introduced, hampering the effectiveness of the set, but it still worked if those users were gotten out of the way. In Gen V, Suicune gained Scald, a slightly weaker version of Surf, with an added 30% chance to burn. This is the preferred move of bulky Water Pokemon now, as they have the bulk to spam it and hope for a burn. In Suicune's case, this made it more difficult for physical attackers to come in and try to take Suicune out, lest they be burned (though it's not guaranteed, the risk is still there) or taking a Calm Mind-boosted one once Suicune has set up enough.
Edit: if you're using this team for 6v6 purposes, you'll have to replace one of your Megas with similar alternatives. If you keep Metagross, I'd use Heatran over Charizard. If you keep Charizard, Excadrill should be used over Metagross, as it has Rapid Spin to clear away Stealth Rock:
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf/Air Balloon
Ability: Mold Breaker
or
-Earthquake
-Iron Head
-Rapid Spin
-Stealth Rock/Toxic
Nature: Careful
EVs: 252 HP/4 Atk/252 SDef
Item: Leftovers
Ability: Mold Breaker
