I have a 45g with a royal gramma in it. I have seen others profiles here where they have both in the tank together. I just want to verify that the gramma will not bother the purple firefish. I have plenty of LR and caves so they should be able to have their own hiding places without much trouble.
I don't think it would be a problem at all in a tank that size. They are both docile fish and as long as there is some LR for the Gramma to hide in, all will be well.