So, after a little research, it appears that Solenopsis molesta would not be a good idea. Unfortunately, there diet consists sometimes of stolen ant brood(so not cool), and that would be bad if they started to hurt the Formica colony. I could imagine that you could use it as a population control possibly, but my colony is nowhere near needing that.
So now I have a new question. I have an Aphaenogaster fulva colony with about 100 workers. Would this colony be able to live with the Formica colony? They now eat mostly almonds(which Formica don't like), so I don't think they would compete for food with the Formica colony. Also, when I first made the terrarium there was an Aphaenogaster colony that had snuck in through one of he rocks. They managed to live with my Formica colony until I dug them out and killed them. I even saw the two ant colony's workers walking right next to each other and there was never any aggression.
I'm currently baking a piece of wood that has lots of tunnels in it for the Aphaenogaster colony, just in case I end up moving them in. Maybe having them live in a piece of wood would keep the two colonies more separated.