I'm sorry for the late response. I remembered your question as something entirely else and thought that I didn't knew the answer.... but this one is easy.

Each unit has a skeleton. Each skeleton has a set of animations, including a charging one.

A single skeleton can be used by multiple units, for example all slingers are using the fs_slinger_new skeleton and thus all have the same charging speed.

In theory it should be possible to give each unit its own charging speed/animation by making an unique skeleton (and thus set of animations) for each unit. Wether R:TW likes that, I don't know.

Does that answer your question?