I think you are overthinking the pest component. If it's in your sump it's probably in your display or overflow. Just ensure that nothing gets to plague proportions and you will be fine.As far as I know, they are all good until they get too big. Do you think they can go from the refugium to the display tank? Also I want to keep frags in the refugium when newly fragged how can I keep it here then?
If you want to run a frag rack in your sump, it will be challenging to do in the same chamber as the algae. The lighting would probably be different and they'd both be competing for space. Possible but I'd use a separate chamber