Give it time. My cons bred in a 20 high with sand, so I don't think sand is a problem. After about 3 weeks my pair formed out of the group of 5. I removed the rest because of bullying and my pair breeds every 3 weeks or so. Mine were slightly bigger than yours.
Pink belly's are the females. They will get there color on there bellies at or around .5''. Ummm.... at a small size, the females tend to grow faster. So your biggest might be a female.
alright so as long as its a convict it will work... lol, i think all 4 of mine are males.... i see no pink on the belly just a lil in the cheek kinda..i guess im gonna have to take a trip to my LFS again.. is there anything else i should get while im there?
i was messing with ya. you can get one if you want though. but on another note it might be more practical to take out the plants. it'll be easier on you keeping the tank clean so maybe a nice piece of driftwood will be better.