i hope your doing water changes daily with that 20G, because it's literally how much you have to do to keep those waste levels down with over 4-6 lionheads in there. the fact of the matter is, your putting them in there temporarily over the winter whereas the OP wants to properly care for them over a lifetime (which in a lionheads case is about 8 years). and although fancies don't get as large as their pond goldfish cousins, they still get pretty big and not mention wide. whats so bad about suggesting a larger tank than is necessary? (btw, i don't agree with the fact that 30G is too large for a lionhead, if anything that size is the perfect for them and shouldn't be any smaller). a larger tank means room for your lionhead to swim and rummage in the substrate (lionheads are quite active fish and although i don't have them, one of my friends do and it's incredibly fun to just watch them swim around) not to mentions waste doesn't build up as fast as they do in an inadequate 10-15G tank.
i've done a lot of research on ranchu's (japanese variety of the lionhead) since i want to keep them one day.