Last time I tried, animations broke, or the game crashed

if I recall someone did do it as they made the gensec swats replace all the default heister models but the problem was they had no heads, it worked but it was not updated for john wick nor hoxton so the game would crash if wick or hoxton walked into the players view but once the game crashed the game could be opened again yet the model now would just be a black texture.

It is possible but It would be best to replace only one heister model like jacket, jiro, bodhi or clover and bonnie since they don't share models with anyone.

Only Bonnie and Clover have separate model files

They do? I thought jiro, jacket and bodhi had their own models since they are not the same as the other heisters.

Do you mean objects inside a model ?

Because bonnie and clover have seperate model files, out of ease for their hair/body.
Jiro, Jacket, bodhi share the same model file, but as separate objects.

It would be easier to replace bonnie or clover, since they are the only ones in their respective model files.