Thats a general fglrx problem that you can not stop X and unload the kernel module. It works sometimes but you definitely can not rely on it.
Thats the case since years, so it would be just lost time. Also switching from oss to fglrx usually crashes the system - even when you unload all modules - i can switch from nouveau to nvidia binary, but not possible from radeon to fglrx.
A majority don't care about that. We generally use one or the other. Using a mixture of both doesn't really benefit us.