Just the cardinals? Maybe for another tank. I could see possibly a 5th fish later down the road but think it will come down to growth and nutrients available.3-4 pj cardinals!
Could just keep it simple, gsp on the middle of the live rock and let it spread to the back glass and across the rocks. Then just chaeto floating in the 20.
Worst case if the nutrients run low and the dt is fully stocked I’ll put a small nano bottom dweller or two in the 20. Might make it harder to sustain a copepod population but seems to be the simplest option. Sounds better than crowding the display tank to sustain both corals and macro