ok i have 20 gallon tank. the tank is moderately planted and has some driftwood hiding areas. i have 5, 2 inch mollies with about 3 fry, some kuhli loaches, a two inch kribensis cichlid and some endlers. will the dwarf puffer attack these fish even if i add more plants?

The guppies and the mollies won't be safe at all. The loaches might be okay since they're bottom-dwellers. Kribs, maybe okay. But since you have the mollies and guppies, I'd say no, probably not a good idea.

Your tank is already way overstocked, I'd see about re-homing some of your fish, in addition to not getting more - tanks with livebearers can turn into a constant re-homing task.