I wouldnt bother doing a RTC and then getting rid of it at 12". You will barely have the thing before you have to get rid of it. Try a smaller type cat like a 4 Line Pim. They get 8-10" and you could keep it for life in that tank. As for cichlids some that I like and am looking at for my 210g are a pearsei or bocourti. Both considered more gentle giants. And my dream fish an Argentea. The 24" width of the tank will get the arrow lovers out there mad at you, most recommend a 30" or wider.
Paratheraps Zonatus
Ex Cichlasoma istlamum
Ex Cichlasoma pearsei
Ex Cichlasoma Festae..
You have a lot of options..
dovii pair all the way or a studly male umbee. unless you want a community where id do a lyonsi trio, la ceiba yellow head pair, red istlanum pair, and salvini pair.