Help with some rom editing please!

[EliteBlitzer]

Hello,Im trying to edit hg/ss rom to make it so that we can battle sinnoh and hoenn gym leaders along the story too,just for the sake of it.I want to ask a series of questions:

1.Has anyone done such a edit? If so could u tell me its name?

2.Is there a noob friendly guide or tutorial for doing such things?

3.If it is possible is there a way to change the music when battling a hoenn gym leader for example to be the gym leader battle from RSE?(Aka editing the game music)

Thanks.