You're welcome Richard.
The Fishman is for an arch top. You can always sand it down to fit a flat top though. I used a sanded down arch top bridge on my electric mandolin (the one with the humbucker).
It all depends whethere you're after a more 'electric' sound - the humbucker is a great thing for that. If you use one on a hollow bodied mandolin, though, you might well still get feedback problems, unfortunately.
I've tried vaious combinations over the years - could you let us know what you eventually settle on and how it works out?
Best of luck with it!