I can't speak specifically for FISH following IF, but I have done them sequentially. Additionally, I have restained tissues that were mounted with prolong gold (adding a NeuN stain to brain slices for example). I don;t see why it wouldn't work. Give it a try! NOTE: Use warm water to heat the slide and carefully peel back your coverslip (this is where your tissue is most likely going to get ruined). I recommend using fine forceps to gently lift a corner of your coverslip and allow liquid in to facilitate an easy cs removal.

Thank you Elliot!