You can only Mega Evolve 1 Pokemon per battle, so you'll have to pick between Beedrill or Sableye.
Here are better sets for Mega Sableye to use, a mono-Calm Mind attacking set, or the tried-and-true support set since Gen V.:
-Calm Mind
-Dark Pulse/Shadow Ball
-Will-o-Wisp
-Recover
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Sablenite
Ability: Prankster
or
-Knock Off
-Will-o-Wisp
-Recover
-Taunt/Foul Play/Toxic
Nature: Impish
EVs: 252 HP/252 Def/4 SDef
Item: Sablenite
Ability: Prankster
If you use Beedrill, Mandibuzz is a fine substitute:
-Foul Play
-Roost
-Whirlwind/Taunt
-Defog
Nature: Bold
EVs: 248 HP/136 Def/108 SDef/16 Spe
Item: Leftovers
Ability: Overcoat
Soul Dew is banned in standard play, so Latios should make use of Life Orb or Choice Scarf:
-Draco Meteor
-Psyshock
-Surf/Hidden Power (Fire)/Earthquake
-Defog/Roost/Recover
Nature: Timid/Hasty
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
or
-Draco Meteor
-Psyshock
-Defog/Tailwind/Hidden Power (Fire)
-Trick
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Choice Scarf
As mentioned by Zekrom, Cloyster's moveset is absolutely terrible. Cloyster was good in Gen V, but not anymore. Priority is pretty much everywhere, courtesy of stuff like Bisharp, Talonflame and Mega Pinsir, making it hard for it to set up Shell Smash. Weavile and Mamoswine are more reliable Ice attackers:
Weavile:
-Knock Off
-Icicle Crash
-Ice Shard
-Pursuit/Poison Jab/Low Kick
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Ability: Pressure
Mamoswine:
-Earthquake
-Icicle Crash
-Ice Shard
-Freeze-Dry/Stealth Rock
Nature: Naive/Jolly
EVs: 244 Atk/12 SAtk/252 Spe
Item: Life Orb
Ability: Thick Fat
or
-Stealth Rock
-Earthquake
-Ice Shard
-Endeavor
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Focus Sash
Ability: Oblivious
Here are better Rapid Spinning sets for Starmie. The former utilizes Reflect Type to have an easier time with Ferrothorn and Bisharp, while the latter uses Starmie's excellent coverage move pool:
-Scald
-Rapid Spin
-Reflect Type
-Recover
Nature: Timid
EVs: 248 HP/24 Def/236 Spe
Item: Leftovers
Ability: Natural Cure
or
-Hydro Pump
-Psyshock/Thunderbolt
-Ice Beam
-Rapid Spin
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb
Ability: Natural Cure
Edit: if you're using Sableye as the Mega, Choice Band Scizor is a fine substitute:
-U-turn
-Bullet Punch
-Superpower
-Pursuit/Knock Off
Nature: Adamant
EVs: 248 HP/252 Atk/8 SDef
Item: Choice Band
Ability: Technician
Edit 2: Bullet Seed > Seed Bomb on Breloom, as it benefits from Techncian, plus 3-attacks + Spore is a better set:
-Spore
-Bullet Seed
-Mach Punch
-Rock Tomb
Nature: Adamant/Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Focus Sash
Ability: Technician
I'd also consider Tentacruel over Starmie, since it somewhat helps with the Fairy problem:
-Scald
-Acid Spray
-Sludge Bomb/Knock Off
-Rapid Spin
Nature: Bold/Calm
EVs: 240 HP/252 Def/16 Spe or 240 HP/252 SDef/16 Spe
Item: Black Sludge
Ability: Liquid Ooze
Other Fairy smashers include Dragalge, Gengar, Scolipede, Heatran and Excadrill:
Dragalge:
-Sludge Wave
-Draco Meteor
-Hidden Power (Fire)/Focus Blast
-Scald/Focus Blast
Nature: Modest
EVs: 200 HP/240 SAtk/12 SDef/56 Spe
Item: Choice Specs/Dragon Fang/Poison Barb/Draco Plate/Toxic Plate
Ability: Adaptability
Gengar:
-Shadow Ball
-Sludge Wave/Sludge Bomb
-Focus Blast
-Destiny Bond/Taunt/Substitute
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb/Focus Sash/Black Sludge
or
-Substitute
-Shadow Ball
-Will-o-Wisp
-Taunt/Sludge Bomb/Sludge Wave/Disable
Nature: Timid
EVs: 28 HP/148 SAtk/80 SDef/252 Spe
Item: Black Sludge
or
-Hex
-Taunt
-Will-o-Wisp
-Sludge Wave/Sludge Bomb/Substitute
Nature: Timid
EVs: 8 HP/248 SAtk/252 Spe
Item: Black Sludge
Scolipede:
-Megahorn
-Poison Jab/Rock Slide
-Earthquake
-Protect
Nature: Adamant
EVs: 68 HP/252 Atk/188 Spe
Item: Life Orb
Ability: Speed Boost
Heatran:
-Overheat/Fire Blast
-Flash Cannon/Hidden Power (Ice)
-Earth Power/Flamethrower
-Stone Edge/Ancient Power
Nature: Timid/Modest
EVs: 252 SAtk/4 SDef/252 Spe
Item: Choice Scarf
or
-Stealth Rock
-Fire Blast/Flamethrower
-Flash Cannon/Earth Power/Toxic/Will-o-Wisp
-Taunt
Nature: Timid/Modest
EVs: 12 HP/252 SAtk/244 Spe
Item: Air Balloon
or
-Magma Storm
-Earth Power
-Taunt
-Toxic
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Leftovers/Air Balloon
or
-Lava Plume
-Toxic
-Taunt/Earth Power/Roar
-Stealth Rock/Protect/Earth Power
Nature: Calm
EVs: 248 HP/192 SDef/68 Spe
Item: Leftovers
Excadrill:
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Ability: Mold Breaker
I dunno who to replace for those, though.