I think it would look more effective with one larger group of schoolers like recommended above, so yeah 12 of the cardinals or rummynose.
I also like the cories just because they add so much activity and are great at grabbing food from the substrate.
If you want to add your SAP, maybe set it up with those guys first, then see what happens with your nitrates (and be sure he's not nipping at anyone.) Since puffers are messy you might be full there. If not, then consider adding something else like your ram. (I personally like Bolivians better than Blues - more hardy, very interactive, and a bit bigger.)
In my densely planted 20 long I had a Schouetedeni puffer (RIP) with schoolers and cories and my nitrates were 0. After he passed away, I added a dwarf pike, a whiptail catfish and a circumcinctus spiny eel. Nitrates still minimal to none - this is a well-established tank though. You'll just have to see what happens.