Getting back into Pokemon... Team help?

EV spreads would be helpful here.

For your current Garchomp set, use Choice Scarf to compensate for the lost Speed that Jolly would have brought.

Walrein is terrible. Don't even bother with it. Its bad defensive typing lets it down, and it's only good on Hail teams thanks to its infamous Stallrein set, which wears down the opponent with a combination of Hail, Toxic and possibly Super Fang; Ice Body in Hail in conjunction with Walrein's Leftovers will give it plenty of longevity. Unfortunately, this isn't a Hail team, so I'm afraid you'll need to take Walrein out. Cloyster is a Pokemon that can function outside of Hail and can go on the offensive with Shell Smash, despite its defensive-leaning stats:
-Shell Smash
-Icicle Spear
-Rock Blast
-Razor Shell/Ice Shard/Hydro Pump
Nature: Jolly/Naive
EVs: 4 HP/252 Atk/252 Spe
Item: White Herb/Lum Berry/Focus Sash
Ability: Skill Link

Or you can use Barbaracle to have a bit more raw power, though its defensive typing is still bad:
-Shell Smash
-Stone Edge
-Razor Shell/Poison Jab
-Cross Chop
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: White Herb/Lum Berry/Focus Sash
Ability: Tough Claws

Ampharos is terrible in the standard environment because of its low Speed, which makes it easy to revenge kill. Wash Rotom is a better alternative:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Pain Split/Hidden Power (Ice)
Nature: Modest
EVs: 232 HP/56 SAtk/220 Spe
Item: Leftovers

or
-Volt Switch
-Hydro Pump
-Pain Split
-Thunder Wave/Will-o-Wisp
Nature: Calm
EVs: 248 HP/28 SAtk/232 SDef
Item: Leftovers

or
-Volt Switch
-Hydro Pump
-Hidden Power (Ice)
-Trick/Thunderbolt
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Specs/Choice Scarf

Houndoom is a good Pokemon in the UU environment, but not in OU because of its Stealth Rock weakness and susceptibility to common types (Water, Rock, Ground, Fighting). Infernape is generally a better option because it's fast and has a wider movepool to take advantage of:
-Flare Blitz
-Close Combat
-U-turn
-ThunderPunch/Mach Punch
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Choice Band

or
-Overheat/Fire Blast
-Close Combat
-ThunderPunch
-Hidden Power (Ice)/U-turn/Mach Punch
Nature: Hasty/Naive
EVs: 180 Atk/76 SAtk/252 Spe
Item: Life Orb/Expert Belt

or
-Fake Out
-Stealth Rock
-Overheat
-Close Combat
Nature: Hasty/Naive
EVs: 252 Atk/4 SAtk/252 Spe
Item: Focus Sash

or
-Fire Blast/Flare Blitz
-Close Combat
-U-turn
-Hidden Power (Ice)/Stone Edge
Nature: Hasty/Naive
EVs: 36 Atk/220 SAtk/252 Spe
Item: Choice Scarf

Lucario's current set is walled by Jellicent. Hard. It has a wide movepool to take advantage of. A mixed set isn't a good idea because it's hard to split EVs for Lucario to take that route:
-Swords Dance
-Close Combat
-ExtremeSpeed
-Crunch/Shadow Claw/Ice Punch/Bullet Punch
Nature: Jolly/Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Life Orb/Lucarionite (ExtremeSpeed can be dropped if Lucarionite is used, since Mega Lucario gained a much-needed Speed boost; regardless, don't use Shadow Claw and Crunch together)

Moonblast completely and utterly outclasses Dazzling Gleam on Sylveon. Dazzling Gleam has no added effects, while Moonblast is stronger and can lower the opponent's Special Attack:
-Wish
-Protect
-Moonblast
-Shadow Ball/Toxic
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers

or
-Calm Mind
-Moonblast
-Shadow Ball/Psyshock
-Substitute
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers

Just saying, this team has a pretty bad Ground weakness. A Ground immune Pokemon like Skarmory is ideal, but I dunno who to take out:
-Stealth Rock/Spikes
-Whirlwind
-Roost
-Brave Bird/Iron Head/Taunt
Nature: Impish/Careful
EVs: 224 HP/252 Def/32 SDef or 224 HP/32 Def/252 SDef
Item: Leftovers/Shed Shell
Ability: Sturdy

Thanks for the in-depth reply.

As far as Garchomp goes, I changed my mind (seeing that I won't be getting a 3DS anytime too soon), and decided to spend my time attempting to breed a good Gible. Currently what I'm going for is a Jolly Gible, with maximum speed IV's and a possibility for maximum attack IV's via past ancestry. Looking towards getting the guy to tell me it's got "outstanding" potential and "can't be any better" in the speed department. Then off to get 252 EV's in speed and attack, and 4 in HP.

As for the team changes, I'm not gonna lie, I'm seriously petty about the aesthetic appeal of the Pokemon on my team. It definitely plays a larger role than it should when I select what I'd like to train – but let's be honest, since I'm not really entering a true competitive scene, I've little to no drive to raise, well, a clam (oyster? I dunno) and a fire-monkey (I just think they're so ugly, man. I know it makes me sound dumb and all, but I just don't think I'd have the patience to go through with training and breeding something I don't really want).

On that note, I definitely like the changes to Lucario and Sylveon. I'll be looking into that once I get my hands on a good Gible. And while I'm talking about them, do you have any recommendations on what IV I should be going for on Lucario and Sylveon? I was thinking of doing the same thing for a Riolu that I am currently doing with my Gible (which is going to blow, because Riolu don't breed…).

As for Walrein replacement, the only thing I can see myself switching with is a Lapras or a Gyarados (which would aid in the whole ground weakness thing). All the other water Pokemon just have no appeal to me. And for Houndoom, the only thing I'd consider in its place would be a Delphox, which also has considerable speed, and could serve as a special sweeper. Again, that monkey… -shudder-. I just can't do it, regardless of that great move pool.

For Rotom, my general issue is again, it's pretty gross looking. That, and all the other electric types I see look pretty terrible as well. What about a Metagross with thunder punch?

Gyarados can helps with Ground weakness, as you mentioned if you don't want to use Wash Rotom. Lapras, despite its wide move pool, isn't cut for standard. Its offensive stats are only average and its defensive typing is the same as Walrein's, which is terrible:
-Dragon Dance
-Waterfall
-Bounce
-Taunt/Substitute
Nature: Jolly
EVs: 88 HP/248 Atk/4 Def/168 Spe
Item: Leftovers

or
-Dragon Dance
-Waterfall
-Bounce
-Stone Edge/Earthquake
Nature: Jolly/Adamant
EVs: 4 HP/252 Atk/252 Spe
Item: Lum Berry/Life Orb

I'm a bit iffy on having Delphox in an OU team. Think about why you'd use it over things like Heatran or Volcarona. Still, if you want to use it, be my guest:
-Calm Mind
-Fire Blast
-Psyshock
-Grass Knot/Shadow Ball/Substitute
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Life Orb/Leftovers
Ability: Blaze

or
-Fire Blast
-Psyshock
-Shadow Ball/Grass Knot
-Switcheroo
Nature: Timid
EVs: 4 HP/252 SAtk/252 Spe
Item: Choice Specs/Choice Scarf
Ability: Blaze

Remember that you get a free Lucario from Korrina at some point in X/Y, while baby Pokemon found in the wild tend to have 31 IVs in at least 2-3 stats, so I'd say 31 IV in Speed and 20-31 in the offensive stat you intend to use. For Sylveon, have around 20-31 in the defensive stats and Special Attack (though 31 would be preferred, you can just go for 20-30 if you can't be bothered going for perfect IVs all round).