Are Raynox magnifiers (125, 250, 5320) compatible with Canon 100mm f/2.8L? In particular, are they going to be compatible with the Canon Macro Twin Flash?
As long as the thread size of the magnifier and the lens match, there will be no 'compatibility' issues - it's just a screw thread after all.
The flash is a different matter however. If the magnifier is a straightforward screw on type you shouldn't have a problem, but if clips on you may face difficulties.
Personally I wouldn't use a magnifier filter in any case: I would use extension tubes.