Aye, the recurring fee handler in core is simply a default one to work with the credit module. The other modules will need their own handlers if they're going to handle the fees, and in that case you'd enable them in the recurring fee product feature settings as valid payment methods.

I guess where the pinch is is that right now we use a single handler to process recurring fees. I guess that means you'd need a custom module to route it to the correct one depending on their payment method selection... which is unintended.

Good feedback, though... will have to work that out as we go. Thoughts? I wasn't intending to write a hook to declare fee handlers, but maybe I'll have to so you can choose a different one for every enabled payment method? Oof.