Do not mix any other species together. Bad news. One or the other or both will eventually die.

It's hard to get what you mean about the tank without a picture. You definitely don't want moss underwater.

You could use silicone and glue some rock to it or use Great Stuff covered by silicone, that all requires removing it from the tank and several days of curing before it goes back into the tank.

You don't have them in this tank yet do you? They will stress easily from messing around with the tank. You should have the tank running properly for a few weeks before putting them in.