Alright, I'm probably going to piss you off a bit.....
BUT....
I wouldn't suggest that you get ANY more fish at all!!!!!! You already have FIVE firemouth cichlids in a 23 gallon TANK.......woah man....no matter if you had 20 filters on the darn tank that just simply isn't enough room for those guys!! I also noticed you have a RTBS also. I think you should maybe just slow down a bit.....you've got PLENTY of fish for that small of a tank!
My advice would be just plain and simply DON'T get anymore fish until you can get a bigger one for all them there cichlids you've got your hands on!!!!
THEN and only then would I suggest getting more fish, and I wouldn't suggest Rainbows either if you are looking for top dwellers. I have some Melanotaenia Rainbows (New Guniea Rainbows) and they are also ONLY mid dwellers to slightly bottom.....they do not swim at the top either except to snap at some food and they don't even like to do that!
Although dwarf gouramis would be nice and they do stay at the top quite a bit.....but only get them when you have room for them
Sorry mate
Just MHO....take it or leave it!