Well, I've decided I want to turn my 20 into a DP tank, when the GSP's are ready for the 60 in 6 months. What other fish should I put in with 4 DP's, and 4 ottos?
K you don't want any fish that are either aggressive or have largish/wavey fins as the puffers will just thank for providing them with a game.
You can put Bristlenose plecos in the tank without too many problems. If you can find them apparently (there is some debate on this) you can get freshwater bumblebee gobies which get along quite well with dwarfs. I keep banjo catfish quite successfully with my colomesus (SA) Puffers but I think the dwarfs would be too small and would probably get eaten... hence you'd lose 2 fish (my ottos seem to have diminished since getting the banjos). Have a read around here there are a few posts on compatibility. The trouble with puffers is that you pretty much need a species tank so you're very restricted with tank mates, saying that though puffers have so much personality that you don't really need many other fish.
In a 20 gal tank you could definatley consider more dwarfs as long as you have enough caves/hiding spots and that you've broken up the line of sight in the tank to stop aggression.
