If you like the firefish and pgmy you can probably add them whenever you want. I like adding fish in pairs to my main display. Two new kids on the block is easier than one new kid on the block.
Filefish can be a bit shy, hence the reason I like adding a pair lets them be more comfortable. You could do a pair and the pgmy.
I'd say any fish your girl (daughter, wife, girlfiend) likes and it fits in your tank would be a major plus in my opinion.