I know you wanted dwarfs, and dwarfs are lovely....but I think you would be happier with one medium sized puff...I have dwarfs, and to be honest my medium sized puffs are a little bit cooler..SHHHH don't tell my dwarfs!
If you want an active puffer for your 20
you could do
http://www.**************.com/forum/ug.php/v/PufferPedia/Freshwater/A_Modestus/
http://www.**************.com/forum/ug.php/v/PufferPedia/Freshwater/A_silus/
or a little less active but still fun
http://www.**************.com/forum/ug.php/v/PufferPedia/Freshwater/T_Turgidus/
or pretty un-active but cute as a button
http://www.**************.com/forum/ug.php/v/PufferPedia/Freshwater/T_Miurius/
or if you are willing to do light brackish
http://www.**************.com/forum/ug.php/v/PufferPedia/Brackish/T_Biocellatus/
These that I mention are fairly popular within the trade and at least one of them should be on that list