Id leave out the mystery snails, theyre useless snails as they dont eat much algae and will eat plants if you have any, plus they do better in cool water than tropical. Mystery snails do better in ponds or gold fish tanks, if you want snails that eat a ton of algae go with nerites, plus nerites come in lots of different colors.
As for an angel fish. Angels do better in pairs or groups plus as has been said they can get up to six inches and as cichlid are territorial and can get a little mean.
If you are looking for colorful fish that dont get big and arent as finicky as angels you could try Rams or apistogrammas then again they can be territorial as well but dont get as big.
As for gouramis i know nothing about them other than they are dumb stupid and boring lol