Hi.
I would recommend you to get another red, 3 is a good number, I wouldnt try 2. 3 maybe 4 but not 2.
Let say that you decided to keep 3, you'll need at least 55 gal. tank to keep them comfortable for life.
Redbellies are in fact very resistant spicies, just keep the water clean, make sure you have a good filtration system that can filtrate at least 3 times the whole tank per hour, remove all the waste that weren't remove for the filter and you'd have a good water quality.
You can set the temp in a range within 27 to 30 celcius degrees the warmer the water is the hungrier they become and the faster they grow up. On the other hand they become more agressive at each other in warmer conditions. I keep my shoal at 29 and despite their constant trying to fight they are thrieving pretty cool and have very nice colors.
Just cycle your tank abouth 3 weeks before getting the reds. Aqcuarium salt addition is recommended aswell.
Cheers.