J. transcriptus form monogomous pairs. As youngsters, it's impossible to tell the difference between males and females, so you'd be better off to get 4 juvies, allow a pair to form, then pull the outsiders. They are secretive about breeding, and most likely you won't know they've bred until you see the fry swimming around on the rocks.
As adults, females are generally larger than the males. They guard newly freeswimming fry, but otherwise only their territory. Older fry help take care of younger fry, and all fry are allowed to stay until their parents kick them out. I have a few 1'' fry with my pair right now and the parents still let them hang around, so they can stay with the parents for a while.
These guys are my personal favorite fish, so I am of course going to say.... GO WITH TRANSCRIPTUS
Also, for the tangs, you may want to add a little baking soda to your water, a pH of 8.0 is the bare minimum... 8.4-8.5 is better.
This tank is too small for firemouths or convicts, both require a 29g minimum for a pair, preferably a 30g long.