Champion Steven has a level 57 Skarmory, a level 57 Claydol, a level 57 Aggron, a level 57 Armaldo, and a level 59 Metagross. In the rematch, he has a level 77 Skarmory, a level 77 Claydol, a level 77 Carbink, a level 77 Aerodactyl, a level 77 Aggron, and a level 79 Metagross.
Steven Stone is the Pokémon League Champion in Pokémon Omega Ruby and Pokémon Alpha Sapphire and his party will consist of a Level 57 Claydol, Level 57 Skarmory, Level 57 Aggron, Level 57 Cradily, Level 57 Armaldo and his final Pokémon which is a Level 59 Metagross that is holding a Metagrossite and therefore will be capable of Mega Evolving. For the rematch after you have beat him once he will have Skarmory, Claydol, Carbink, Aerodactyl and Aggron all at Level 77 while his Metagross (which still holds its Mega Stone) is at Level 79.
There is no roller skate item in Alpha Sapphire.
You can catch Skitty in Pokemon Omega Ruby and Alpha Sapphire in the barks of trees.
You battle Kyogre for your chance to capture it in Pokemon Alpha Sapphire after you capture the groudon.
Magikarp
Yes, Tm's in Pokemon Omega Ruby and Alpha Sapphire are re-useable
No, you will need to trade for them or transfer them via Pokemon Bank to get them in Pokemon Omega Ruby and Alpha Sapphire.
Houndour can be obtained by trading from Pokemon X or Y. It cannot be caught in the wild in Pokemon Omega Ruby and Alpha Sapphire.
You will need to trade for Weedle or breed Kakuna. Kakuna can be found only in the Safari Zone in Pokemon Alpha Sapphire and Omega Ruby.
No
It is not yet known if there is a building location for the GTS in Pokemon Omega Ruby and Alpha Sapphire.
Yes, all the TMs that were in Pokemon X and Y will be available as TMs in Pokemon Omega Ruby and Alpha Sapphire.
No, there are no roms for Pokemon Omega Ruby and Alpha Sapphire currently nor a working 3DS emulator. (They are also incompatible with the Desmume emulator.)