Even the smallest dwarf species like a fair bit of space, and I wouldn't do one of the larger species (flame, CB, Potter's, halfblack, lemonpeel etc.) in anything less than 50g, preferably more. I keep a C. flavicauda, one of the 'dwarf dwarf' Centropyge angels, in my 30g and even that's pushing it IMO. I have a coral beauty, a flame and a cherub in my 65g and they use every single inch. I can't imagine a flame being too happy in a 20g, but it's possible people have done it with success.