I have a 20 gal high tank. Nothing special. I have a false percula clown and a neon damsel. Just wondering how many more fish i may be able to keep in the tank. Was looking at a black false perc.
You could do the black ocellaris, then you would be at capacity, IMO. If they are about the same size, there may be scuffling, so I'd suggest getting the new fish smaller, if you can. There will still probably be a little chasing, but your current fish will be dominant and they'll settle down. They really seem to be more comfortable as pairs, so your plan sounds like a good one.
It is a small tank and both fish you have can be aggressive to new arrivals. Make sure the new fish has hidding places( going to be hard in a small tank) and watch closely for the first few days. If the new arrival is hidding in an upper corner after 2 days I would remove it.