Thing with ottos is they’re hard to keep anyway. So if you have 3 doing well in a 20 I would let them be. If you’ve got them eating algae wafers or vegetables than you could try more. They are mostly if not only wild caught. So any new ones should be in a qt tank for a month before added to main tank.
Me I would add another school of small tetras. Again qt if you can.
I don't know how long you've had the rams, but I'd watch out some (not all) can be territorial. For safety sake when you add any fish to your 20G move some of the deco around so the rams don't get protective and aggressive with the newbies. You could also try a half dozen rummy-nose tets - active, pretty and docile.