For flat fish like sole that tend to be rather sticky, either butter your foil very generously so that the skin doesn't stick, or line the foil with something like fresh leaf spinach, or lamb's lettuce.
It just stops the fish sticking if you wan't to avoid using loads of butter.
I have done frozen fish fillets in foil parcels, yes, it is fine but of course make sure you make the cooking time long enough. put some oil pref olive on the square of foil,place the fillet on this. squeeze over lemon/lime juice,prhaps sprinkle some herbs eg oregano, some salt and pepper. Essentially you can make it up yourself a bit, that's the great thing. If you are using frozen fish, then long slowish cooking.