I would suggest you don't get moonlights. These get to 6" so putting two of these and two pearls in a 20 gallon is going to mean a lot of territorial disputes for the upper layers. Instead, if you want variety, why not try some of the colisa species? Not the dwarfs (colisa lalia) but maybe colisa labiosa or colisa fasciata both of which reach a maximum of around 4" or some honey gouramies (colisa chuna/sota) which get to about 1.5-2". Croakers and sparklers are very similar in appearance and temperament with only a slight difference in color. Both or either would work fine in your tank.
I would suggest you get 2 pearls like you had planned, the croakers or sparklers like you had planned and also put in 3 honey gouramies and one of the larger colisa species (either labiosa or fasciata) that I mentioned instead of moonlights. Obviously it is up to you though and the moonlights WOULD work it's just that there's a chance there'll be confrontations. Try to get only a male pearl with a female pearl and female moonlights if you go this ruite. This will decrease any aggression.
Remember to provide lots of cover for all the gouramies to retreat to. If you decide you want something for algae, I reccomend bulldog plecs and bristlenose plecs. American-flag fish are also good algae-eaters and would make a very interesting addition. I'd only put one male in a 20 gallon though as they are territorial and similarl behaviour-wise to gouramies.
