Well I don't Know about 150 gallons. With two Festae chances are 150 gallon isn't big enough for any other fish and if they are two males it's bad but two females it's worse. I would say 6x2x2 is your best bet. Even then it will be sketchy. Yes it's a big step for a 55 but if you want to keep the fish it's the right thing to do. Severums might be toast when the Carpinte and Festea get bigger.
Problems with reintroducing fish I have noticed if there is a set picking order in the tank and you add a re-intro it screws it all up as now lower pecking order fish are asserting dominance and it's just a fiasco of dominance.