I didnt replaced anyone, i keep the gallean beast as a limit break, and changed the animation script of all vincent's limit so they are not AI controlled, then i changed the animations script of each model ( chaos, gigas, gallean and hellmask) so they can perform moves like " double cut" " death blow" even the standar attack as to be modded, to summarize, there is 2 step for this.
But if i understand well, in your case you need to replace red13 model with gallean beast, then u need to put your werewolf on the gallean's skeleton model, then editing it with hex editor to change his animation script so they fit with a playable character, the problem is Im not sure if the values are the same for PC and PSX.
Example of an animation script for playable character: when u open it with hex editor
E8 B6 00 AA 01 02 03 EC E5 EE
E8 mean the character load an effect (magic , summon etc), B6 mean the camera is paused, 00 mean to wait, AA unpause camera, 01 play the first animation of the character, 02 second animation etc, EC will launch the magic itself, E5 reset your character position, EE is the end of the animation script, knowning all these values ( and more) you can customize each animations of a character, even bosses and monsters( but since they are not playable there is more works to do on them)
Once again, all these infos are for the PSX versions, i will check if the PC version are similar,but never modded on pc and dunno where the battle models and vincent limits models are located.