One way you can catch Pikachu in either Pokémon Omega Ruby or Pokémon Alpha Sapphire is that you can find one to catch in the Safari Zone additionally you could have someone else catch one for you and trade one of your common Pokémon for it and you could also use one of your previous Pokémon games and catch Pikachu in one of those games and then transfer it over into either your Pokémon Omega Ruby or Pokémon Alpha Sapphire game.
You can catch Skitty in Pokemon Omega Ruby and Alpha Sapphire in the barks of trees.
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.
These Pokemon can be found before the National Dex: Latias (Alpha Sapphire), Latios (Omega Ruby), Kyogre (Alpha Sapphire), Groudon (Omega Ruby), Regirock, Regice, and Registeel. These Pokemon can only be obtained in the Delta Episode: Rayquaza and Deoxys. Post-National Dex Legendary Pokemon are: Regigigas, Ho-Oh (Omega Ruby), Lugia (Alpha Sapphire), Heatran, Mesprit, Uxie, Axelf, Palkia (Omega Ruby), Dialga (Alpha Sapphire), Giratina, Cobalion, Terrakion, Virizion, Reshiram (Omega Ruby), Zekrom (Alpha Sapphire), Kyurem, Raikou, Entei, Suicune, Tornadus (Omega Ruby), Thundurus (Alpha Sapphire), Landorus, and Cresselia.
No
It is not yet known if there is a building location for the GTS in Pokemon Omega Ruby and Alpha Sapphire.
The ability of Mega Diancie in Pokémon Omega Ruby and Alpha Sapphire is Magic Bounce.
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.)
No. Pokemon Omega Ruby and Alpha Saphire are remakes of Ruby and Sapphire, from 3rd Gen. Omega Ruby and Alpha Sapphire are in the same generation as X and Y, 6. Hope this helps you.^w^
all of them
There is none