Dwarf puffers are fine as singletons in their own tank. Some of them have a nasty enough personality that they won't tolerate other puffers (or even other fish) in their tank, while others seem to enjoy having company.
I have a tendency to project human feelings onto most of my fish, but especially my puffers because they are so personable and interactive with people, and I always feel that they need another fish to keep them company. Over time I have learned the hard way that if a puffer doesn't particularly want company they are quite capable of removing the new occupant with extreme prejudice. As long as your Dwarf is not "sulking" in the corner all day, and is relatively active, there is no real need to get it some "friends"..
If you are looking to add tankmates for "cleanup duties" then Ottos are a good choice for the smaller tanks, or even Algae eating shrimp can make good tankmates (they may end up as food for the puffer but they get some good cleaning in before they turn into a snack).
Or, let him be in his 5 gal; where fresh water puffers don't live too long (as I've heard) anyway?
If kept in the right conditions many FW puffers are reported to live for 15+ years. I'm not positive but I believe that Dwarves would fall into this catagory as well.