In my little book "the Interpet Q an A manual of the marine aquarium" it says that dwarf angelfish should be kept in a minimum of 25 gallons. They also need high quality filtered water whcih means a protein skimmer and activated carbon.
Yeah I would not recommend going saltwater unless you have a minumum of 40 gallons. The bigger the better..
smaller marine tank parameters also fluctuate too greatly for fish to cope with. You could build a nano reef (check out some s in google) but I wouldn't recommend them without some previous experience.
They're beautiful tho!