Use the tool at /kkzero241/AT7View on github (GBAtemp won't let me post links)
Music files are the same format (or similar at least) as the Explorers games, not regular brstm Wii music files
Edit: I can't answer your other questions, sorry. Maybe you can find something on Project Pokémon
Which Pokémon do you want as your partner and starter? Normally you can choose all of the starter Pokémon from the main games plus Pikachu or Riolu for your player character and partner (the partner can't be of the same type as the player character though), but feel free to choose anyone else...