We assigned (actually, that may even be "I assigned", but it was a long time ago and I don't remember) the .mdl extension because it was an obvious abbreviation of "model", not because of any connection to anyone else's format. Unless Quake uses arbitrarily mixed triangles and quads, and stores the skeletal rig as a series of joints rather than bones, they're unlikely to be a match.
I can't remember the exact texture size CC uses (it's fixed, may be different for overworld vs. combat models, PCs always have one texture only, some NPCs have more), and have no idea whether the size was mandated by the hardware or arbitrarily selected by Square. Someone at romhacking.net might be able to point you in the direction of better hardware information for the PSX, though.
As for increasing the size of the party...yeah, you're in uncharted territory there. Little work has been done on the program executables--we know that they're the files at the beginning of the disc, and a couple of the routines have been tentatively identified, plus some people have found pointer forests or other embedded bits and pieces, but that's it. Successful modification would probably require a knowledge of PSX assembler and/or a debugging emulator, which is out of my scope at this time.
