Sorry to disappoint you

but Angelfish don't mix well with Gouramis. Plus, a 20 gallon tank is really too small for Angelfish (they need at least 30 gallons of water). But there are plenty of other beautiful and smaller fish

Which kinds are you particularly partial to? There are many small and beautiful Tetras, Rasboras, Barbs, etc ... that you can choose from.
You already have 3 Dwarf Gouramis in your tank, which are a total of around 6" - 7.5". That leaves you with additional 14" - 12.5" of space. You could definitely add to your tank a beautiful school/group of some small fish. Some examples are Neon Tetras, Glowlight Tetras, Cherry Barbs, Harlequin Rasboras, Zebra Danios, White Cloud Mountain Minnows, etc ...