Greninja should use Hydro Pump over Scald, and doesn't really need Taunt. Your team is spreading enough status as is, so Scald's burn chance will probably go to waste. in place of Taunt, you could use, Grass Knot, Spikes, Hidden Power, or something.
Gourgeist should use Shadow Ball/Shadow Claw instead of Phantom Force. It's cool and all (and I'll admit I'm guilty of keeping Shadow Force my on Giratina-O), but its easy to deal with since it has a set-up turn.
Rapidash doesn't need Poison Jab; it hits Fairies hard enough with its other attacks (especially Flare Blitz) and Poison Jab doesn't offer any extra coverage. But Low Kick (or was it Low Sweep?) does, against Rock types, which its other moves don't deal with so well. So go with that, if its not too much trouble (chances are you won't need it that much and will be spamming Flare Blitz).
Yeah you're right, maybe Taunt is way overkill, will Hidden Power Dragon do much?(Just in case of Mega Charizard X). I also have to agree with you about Gourgeist and Rapidash, Phantom force is really cool, but it's set up bait, I might consider Shadow Sneak. Well I better breed Rapidash with Low Kick, since Megahorn will most likely miss, and with Low kick Rapidash can K.O. a Tyranitar. Anyway thanks for pointing that out!
Rapidash isn't that good. Seriously. Because of its shallow move pool and the fact that Poison Jab is redundant with Flare Blitz, while Megahorn has no noteworthy coverage. Arcanine is the superior choice because of its better stats and movepool:
-Flare Blitz
-Extreme Speed
-Wild Charge/Crunch
-Close Combat/Morning Sun
Nature: Adamant
EVs: 72 HP/252 Atk/8 SDef/176 Spe
Item: Life Orb/Expert Belt
Ability: Intimidate/Flash Fire
Gourgeist is commonly seen as a physical wall, so it may be a better idea to take advantage of its physical Defense. Quick Claw is frowned down upon competitively because it's dependent on luck:
-Will-o-Wisp
-Leech Seed
-Protect/Pain Split
-Seed Bomb/Shadow Sneak
Nature: Impish
EVs: 252 HP/4 Atk/252 Def
Item: Leftovers
Ability: Frisk
Actually, I'm also considering Arcanine as an alternative to Rapidash since it's a beast. And Will-O-Wisp is there to cover Gourgeist's Physical Defense, and is Big Root a good alternative just to abuse leech seed? Because I don't quite like Trevenant even if it's 50% on Harvest, I'd rather use him in a sun team. And I'm considering to replace Protect with Pain Split since she is slow.
Anyway, thanks for the suggestions and pointing out the flaws!! I'll consider the suggestions!! Thanks again!!