just my experience but....
I have a sump for my 75gallon tank, I would say mod planted with 25 rainbow fish. I feed flakes 2-4 times a day, whiteworms or brine shrimp 1 x a day. my sump has 1- 4" coarse poret foam sheet, 1- 2" fine poret foam sheet, the foam filter that came with the return pump and some wasted purigen(wasted because I think I get very little flow through it) and a mag 5.0.
I could have 10 sheets of poret foam and bio rings and bio-balls and what not, but all that is really just wasted. This works fine and simple. No baffles needed and probably
5X cheaper in the long run.
As far a nitrates I would just say more plants growing better in your tank should fix that. If it doesn't then try to figure out why and address that. If you are set on the refugium idea then make your sump able to grow terrestrial plants they are better at nitrate removal.
20L My Other Tank: OEBT Shrimp
40B Inlaws Tank: 2 goldfish
75 Main Tank w/ sump: 5 Glossolepis pseudoincisus, 5 Melanotaenia boesemani, 10 Melanotaenia praecox, 5 Melanotaenia lacustris, some cories, a bn pleco and rainbow fish fry