Aggron is terrible in OU without its Mega Evolution because of its base form's bad defensive typing, which gives it very common weaknesses to Ground, Fighting, and Water (and the former two are 4x weaknesses). The better Steel Pokemon to turn to are Ferrothorn, Skarmory, or Heatran:
Ferrothorn:
-Stealth Rock/Spikes
-Leech Seed
-Gyro Ball/Thunder Wave
-Power Whip/Protect
Nature: Relaxed
EVs: 252 HP/88 Def/168 SDef
Item: Leftovers
Ability: Iron Barbs
Skarmory:
-Brave Bird/Counter
-Whirlwind/Taunt
-Spikes/Defog
-Roost
Nature: Impish/Bold
EVs: 252 HP/252 Def/4 SDef
Item: Leftovers/Rocky Helmet
Ability: Sturdy
or
-Iron Head
-Spikes
-Roost
-Whirlwind/Taunt
Nature: Careful
EVs: 252 HP/4 Def/252 SDef
Item: Leftovers
Ability: Sturdy
Heatran:
-Lava Plume
-Stealth Rock
-Toxic
-Taunt/Protect/Roar
Nature: Calm
EVs: 248 HP/192 SDef/68 Spe
Item: Leftovers
I wouldn't call Mega Gardevoir a special tank. Its stats are more offensively-leaning, even though a massive Special Defense is a nice bonus. Your set for it is spot-on, but the nature should be Timid or Modest and the EV spread should be 24 Def/232 SAtk/252 Spe to take 2 Psyshocks from Latios.
Meganium is a terrible Grass Pokemon in OU. It doesn't have much to set itself apart from other Grass Pokemon like Amoonguss and the aforementioned Ferrothorn. If a defensive pivot is necessary (and the above Steels I suggested are support Pokemon), Wash Rotom should be the one to do this, as Volt Switch in conjunction with its low Speed lets it bring in its teammates unscathed into battle:
-Volt Switch
-Hydro Pump
-Will-o-Wisp
-Pain Split/Rest
Nature: Bold
EVs: 248 HP/252 Def/8 Spe
Item: Leftovers/Chesto Berry
Gardevoir Pursuit support from Weavile, Bisharp, and Tyranitar, as Ghost Pokemon are problematic for it:
Weavile:
-Knock Off
-Icicle Crash
-Ice Shard
-Pursuit
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb
Ability: Pressure
Bisharp:
-Sucker Punch
-Knock Off
-Iron Head
-Pursuit
Nature: Adamant
EVs: 252 HP/52 SDef/204 Spe or 192 HP/220 Atk/96 Spe (latter for Assault Vest only)
Item: Life Orb/Dread Plate/Black Glasses/Assault Vest
Ability: Defiant
Tyranitar:
-Stone Edge
-Crunch/Ice Punch
-Pursuit
-Superpower
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Ability: Sand Stream
Gardevoir isn't that fast either, so Thundurus can provide Prankster Thunder Wave to slow down faster targets:
-Thunderbolt
-Hidden Power (Ice)/Grass Knot
-Focus Blast/Grass Knot
-Thunder Wave
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb
Ability: Prankster
or
-Nasty Plot
-Thunderbolt
-Hidden Power (Ice)/Grass Knot
-Thunder Wave/Focus Blast
Nature: Timid
EVs: 4 Def/252 SAtk/252 Spe
Item: Life Orb/Leftovers
Ability: Prankster
Garchomp should be Jolly with 252 Atk/4 Def/252 Spe. Its item should be Lum Berry, Rocky Helmet, or Focus Sash
