Well it depends a little on what species you get (there are a few sold as 'fire bellied newts'). I've seen
Cynops pyrrhogaster and
C. orientalis advertised recently, and though I haven't checked them out very closely I suspect that both are being sold simply as 'fire bellied newts'. Not that it makes much difference, just that
orientalis are slightly smaller so you might be able to get an extra one.
They are small and hardy, you could fairly easily get 3 or 4 in your 20g. They are very tolerant of temperature and are a cool water species, so you shouldn't need a heater. They are also pretty much aquatic and could be kept without a dedicated land area - some floating cork bark or pieces of wood that break the surface will do - that said, if you want a land area for plants then I would stick with a simple fine potting soil that hasn't had any fertiliser etc. added to it (I assume, I've never kept anything that needed a land area beyond a solid platform of some kind - someone else might have a better idea).
As for substrate it is best not to put anything in that they are in danger of swallowing, and no sharp gravel. I tend to use no substrate for small newts and larger pea gravel and river stones for large newts (baring in mind that my
Pleurodeles are a good four or five times the size of your average fire belly). Floating plants like elodea and water lettuce seem to be the best bets.
Filtration and water changes are much the same as for fish. Hope that helps a little.
Here is an (slightly old) pic of my newt tank.