I think dwarf and pigmy are the same...30g and lots of rock work, they like to swim in and out of it, and they are perfectly happy. They don't require a tank the size of a 55g, but bigger is always better. However, 20g is too small. Remember the 30g is the minimum size tank they can be happy in, folks can get away with 29g.
This is what we are referring to as Pygmy angel. And it does say 35 gallon for this this one too so not a good idea. Angels spend a lot of time swimming and picking at the rocks food. If they are crowded, they will be stressed which leads to all kinds of issues from disease susseptability to aggression. Ultimately death is often the eventual ending https://www.saltwaterfish.com/product-pygmy-angelfish