With 3 platies and a small group of harlequins there is alot of possibility. First of all, keep in mind that you platies will give birth and the fry will become adults, this will take some space. Remember the 1" of fish per U.S. gallon when adding fish to your aquarium. You can have about 2-3'' inches more fish but its highly recommended not to go over (whch is 20''-23'' for a 20 gallon tank in your case). ok now with the fish.
you could add some dwarf gouramis (grow to 2.5'' max), a normal gourami (up to 6'' max), a group of 5 tetras (they can each grow up to 2'' depending on species), a khuli loach (5'' max), 3-4 corydoras ( up to 3'' each), a small to large group of otocinclus' (max 1.5'' each) or balloon mollies (up to 2'') and I personally love them!!
Hope I gave you some insight and remember 1'' of fish per U.S. gallon of water. Cory's, tetras and otocinclus' must be kept in groups. There are alot of other fish and here is a helpful website with many variaties and facts about them all!
Best of luck
www.thetropicaltank.co.uk
