Sorry if I mislead you about keeping only one dp. you can have tankmates but I have come to realise that they do best without other dp's in with them. IMO they aren't too agressive with other types of fish, they will usually harass eachother unless they are given large amounts of space and plenty of cover to break up their lines of sight to eachother. That's not always the case, and people have succesfully kept them together, however my rule of thumb is always only one puffer per tank. other semi-agressive fish who are not big enough to swallow the dp's whole are usually fine with them.
Hope that helps
Good Luck
SLC