Thats about the load I have in my 7foot! I definitely wouldn't be adding anything right now. When some of your fish get to large for your tank and you have to get rid of them, then you could add another??
You're going to be WAY overloaded when those fish start growing. Except for the eel (and even that I'm not sure) EVERY single one of those fish will outgrow a 55 show. Don't add more, you need to reconsider you're fishes ultimate sizes and trade out fish. For example instead of a blue tang (assuming regal/hippo/blue) get a kole--they don't get that big. Forget puffers altogether. Tobies are relatives and very interesting and stay small. Otherwise, you need to upgrade big time--like to a 180.