I kept kuhlis and DPs together in a 10 gallon with no problems at all. 3 dp, 5 black kuhlis (about 3"), and a yellow algae eater. Since the kuhlis are so narrow (like a tube) and they like to congregate in tight spaces the 1" per G doesn't really apply. If you only have one DP I think you should be fine with a few kuhlis.
Note: make sure you know if your DP was bred in the wild or in captivity. If it was the wild the skin can be poisonous and harmful to the kuhlis who will graze on a dead body if they find one. The same would go for just about any other fish kept with a puffer.