Rate My Team?

Probopass @ Custap Berry
Ability: Sturdy
EVs: 248 HP / 8 SpA / 252 SpD
Calm Nature
- Magic Coat
- Stealth Rock
- Pain Split
- Flash Cannon

This Probopass is a suicide lead. It's sturdy would allow it to live a fighting/fire/ground type move, while setting up with stealth rocks or magic coat-ing back other statuses. It's berry lets it use priority pain split to take a chunk out of an attacker.

Sableye @ Leftovers
Ability: Prankster
EVs: 248 HP / 8 Def / 252 SpD
Calm Nature
- Confuse Ray
- Will-O-Wisp
- Recover
- Substitute

I love my Sableye. This set is fairly obvious. I'm only scared to switch Will-O-Wisp for Toxic because of immunities and the attack drop being lost.

Gardevoir @ Choice Specs
Ability: Synchronize
EVs: 248 HP / 252 SpA / 8 SpD
Modest Nature
- Psychic
- Moonblast
- Thunderbolt
- Shadow Ball

Gardevoir is my favorite Pokemon. If I didn't want her to be a Mega, is this an effective way of pulling it off?

Gyarados-Mega @ Gyaradosite
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Dragon Dance
- Substitute
- Waterfall
- Crunch

This Mega-Gyar is meant to set up Dragon Dances while a Support Mon is out, then sweep using its STAB moves.

Venusaur @ Heat Rock
Ability: Chlorophyll
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
- Sunny Day
- Solar Beam
- Synthesis
- Sludge Bomb

Weather is a tricky thing, yet this Venasaur is functional once I can set up. Is it worth the risk?

Magnezone @ Light Clay
Ability: Analytic
EVs: 248 HP / 252 SpA / 8 SpD
Modest Nature
- Light Screen
- Reflect
- Thunderbolt
- Flash Cannon

Screens Magnezone. Nothing too complicated here. Is it done in an effective way?

Thank you so much in advance!

Skarmory is a better Custap lead than Probopass, IMO, as it has Brave Bird to KO itself when the Custap Berry activates to prevent Rapid Spin/Defog from clearing the hazards. However, the Custap lead is a waste of Skarmory's defensive potential. A better hazard lead is Scolipede, since it has higher Speed to get hazards up before most opponents, while Speed Boost makes it more difficult for it to get Taunted by non-Pranksters. It also has Endeavor to severely weaken opponents when its Focus Sash is used:
-Pin Missile
-Spikes/Toxic Spikes
-Endeavor
-Protect
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Focus Sash
Ability: Speed Boost

Your current Sableye set is massive Taunt bait. Regular Sableye is also outclassed by its Mega counterpart because the latter has Magic Bounce to block status moves. The other defensive Ghost I can think of that's viable in OU is Cofagrigus:
-Toxic Spikes
-Hex/Shadow Ball
-Pain Split/Snatch
-Will-o-Wisp
Nature: Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers

However, the only real viable Ghost Pokemon in standard play is Gengar, which can run a surprisingly effective offensive utility set:
-Shadow Ball
-Sludge Wave/Sludge Bomb
-Focus Blast
-Taunt/Icy Wind/Destiny Bond/Substitute
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb/Focus Sash/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

Regular Venusaur isn't worth using over its Mega counterpart because the latter is bulkier and has Thick Fat to negate two of its weaknesses. Using sun without having a good chunk of your other teammates benefitting from the weather isn't good either (for instance, Gyarados' Water STAB will be weakened in sun). Breloom is the other offensive Grass alternative:
-Bullet Seed
-Mach Punch
-Rock Tomb/Swords Dance
-Spore
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Focus Sash/Life Orb
Ability: Technician

or
-Swords Dance
-Seed Bomb
-Superpower/Drain Punch
-Facade/Mach Punch
Nature: Jolly
EVs: 12 HP/244 Atk/252 Spe
Item: Toxic Orb
Ability: Poison Heal

Regular Gardevoir also isn't worth using over Mega Gardevoir because the latter is terrifyingly more powerful with Pixilated Hyper Voices, which is its main niche. Hoopa Unbound is a better Choice Scarf user, since while it's stronger than regular Gardevoir, it's only as fast as Gardevoir. However, it needs a bulky U-turn/Volt Switch user to get into battle safely:
-Psychic/Zen Headbutt
-Hyperspace Fury
-Gunk Shot
-Focus Blast
Nature: Hasty/Jolly
EVs: 252 Atk/4 SAtk/252 Spe
Item: Choice Scarf

You're wasting away Magnezone's main niche of trapping Steel Pokemon and eliminating them with Magnet Pull. Given its low Speed, it makes an ideal teammate for Hoopa Unbound to get into battle safely, as Volt Switch has lower priority than manual switching:
-Thunderbolt
-Volt Switch
-Flash Cannon
-Hidden Power (Fire)
Nature: Modest
EVs: 172 HP/252 SAtk/84 Spe
Item: Choice Specs
Ability: Magnet Pull

Thank you so much for the amazing input! This is the first actually more than permanent team I've ever tried to create. I really appreciate the Into for Probopass, since hazards are important to me. I'll definitely take much of your advice.