Hello again!
Oh that's good you managed to return the rainbow to your LFS. Did they give you a store credit or anything?
When treating with Myxazin there's no need to remove the fish from the tank as it's nothing contagious that the gourami has. That has been caused from stress (from the rainbow and the ammonia). Even 1ppm of ammonia is high enough to cause stress and disease in a fish. Are you doing small daily water changes after testing? Obviously, if you do a test and you get 0ppm ammonia and nitrites then you know you don't have to do a water change that day, although it will probably take several weeks to reach that point.
I know you said your tank is 29gallons (which is roughly 3ft yes?) so I really wouldn't advise you to get any type of the shark family, especially not the balas you mentioned as it's just not fair on them. You will again have issues over territory and then as they grow they will need a much bigger tank - which means more hassle for you trying to rehome them (and stress for the other fish in with them at the time) and stressful for the fish being moved... I just think that when you buy a fish you have to think ahead and if you know you cannot give that fish the home it needs at adult size, then don't buy it. Just my honest opinion. The less stress for the fish and you, all the better! Happy fish = healthy fish!
So, as for other tankmates to go with your couple of angels and gourami's - well, firstly make sure your tank is properly prepared for your existing fish, with plenty of tall plants midway to create that territory divide. Gourami's can be just as territorial as rainbows! Especially as you have two males in there. So try to create a division so that they can each claim an area of the tank once they reach that stage of maturity when they might become aggressive towards each other.
Other fish to add should be fish no larger than the gouramis and look for peaceful species. What about some shoaling fish? e.g. harlequins, or tetras (there are various to choose from but avoid serpae as they can be a bit cheeky and try to fin nip), guppies, how about 5 or 6 sterbai corys (they are fascinating bottom dwellers - their eyes are like a lizard in that they can move them and look around and even blink! I got a shock the first time I saw my sterbais doing this.).
Avoid any more cichlids in that size tank as that could cause problems with your angels.
Regards, Athena