Dwarf Puffers are best kept in a species only tank as they are very nippy. Some people keep them with Ottos but that's pretty much all you can keep them with.
What type of fish are you interested in and what size is your tank?
Are you planning on moving your fish to the new tank or are you looking to start a complete new list?
It really does depend on what type of fish you have in mind and whether you want all the fish to be semi-aggressive.