Grunt Red XIII? That shouldn't be too hard to do on the field, but there are no battle models for him, so you'll have to make those yourself or hope that someone else is willing to make some. i gather that it takes quite a bit of time to make good battle models.
Putting Crisis Core music in the game shouldn't be hard at all; FF7 music can substitue any mp3s you want for the tracks that play in the game. You could make Aqua's "Barbie Girl" play during the Safer Sephiroth fight if you wanted to do so. If you edit the field files, you should be able to customise the music even further. See
this thread for some information on my fumbling attempts in this area.
If you have nine party members at the end of the game, I can't see why you couldn't have three parties of three fighting Bizarro; it should just be a matter of field editing with meteor.
Making new playable characters might be harder. Replacing playable characters with others is fairly straightforward if a little time-consuming, but actually adding ones sounds like quite a task. Someone else will certainly know far more than me about this, as my expertise is mainly in the kernel.bin and scene.bin files rather than the more complex things involved in doing this. So, does anyone who (unlike me) knows whereof s/he speaks have any ideas on the feasibility of this?
