Keeping them in a 300 - 400 gallon tank is not neccesary. You can get away with keeping them in a 210 just like you are. There are quite a few hobbyists that also do it, I even know people breeding them in tanks that size. However, it is not ideal.
Do not doubt the info given on this site. People will usually state that cichla need larger aquariums simply because a 24inch fish will feel more comfortable in a 300-400gallon tank rather than in a 210. The people here are generally just looking out for the comfort of the fish and the general well being
If it is working for you thats great, however im sure your cichla would appreciate a tank upgrade
How big is the RTC? It is quite possible that he will outgrow the tank though....