Quote from Paul;254691:
Why so much legendaries in this thread? o_o
The cat never specified any sort of tier to stick to, so it's only natural some people will go for the heavy hitters.
As for Item Clause, it's been
programmed into the game and is in use during many online battles. Unofficial communities and game clients seem to turn it off, but since you're playing on a DS I figure it's worth keeping in mind.
As for your team. At first glance items and movesets seem pretty alright, and there's no giant weakness among the types. Gardevoir isn't bringing much to the table, though. Gardevoir can fill several slots at once due to his/her weird moveset... but as a Special Sweeper it has the same two sweeping moves your Mixed Sweeper (Gengar) has. Gengar's speed will make up for the lack of Calm Mind and you'll get a lot of use out of him with Sub (hint hint: put the Leftovers here. Or Shell Bell, or something. Healing = more subs = more Gengar ownage). You'll find that with a few EVs in SAtk Gengar doesn't need the 20% boost the Expert Belt is giving him.
This will probably be a testing point of your team.
Metagross, Blissey, Aerodactyl are pretty standard. I guess Hippowdon is too. I think my only concerns lie with the item placement, other than Gengar potentially overshadowing Gardevoir.