A few things to go through here, so will try and go through it all point by point....
1. Dwarf Puffers (Carinotetraodon travancoricus) don't tend to have overgrown teeth problems, and even if fed snails, they tend to suck the meat out of larger ones, and may eat shell and all of only the much much smaller ones...
2. Any puffer can be fed a pellet diet, its just that they may or may not take to it and can be rather messy with them, but definitely no reason why you cannot use a quality pellet food as a staple source, but flakes as far as i've seen, never work for a puffer coz they just aren't interested...
3. Tank mates: Anything with long flowing fins, Crayfish, even smaller ornamental shrimp are a no go imo with dwarfs, cray fish as mentioned could hurt / kill the puffers, and any fish with long flowing fins will definitely get nibbled on... The ornamental shrimp in my own experience has been touch and go, coz once they learn that they can eat the shrimp, then they will start to target them and take them down... Otos on the other hand might work, but may still end up with a few torn fins, same with the pleco...
4. In a 20L tank, i would also probably recommend no more than about 6-8 Dwarf Puffers (if its heavily planted / decorated) making sure to have a max of around 2 males and the rest females to help spread the aggression of the males out between all the females...