Hi,
I have a single Pea Pea Puffer in a 5g rectangle (Fluval Spec). I'll move him up to a 6g when I can, although I'm sure he's hoping for a 10g! I think one in your rectangle is the only option for you. Other choice for rectangle is one betta and maybe a 2-4 nerite snails depending on their size.
As far as the biorb, I've read that they are disorienting for fish. My Puffer looks disoriented already as he tends to waft about. Make no mistake, when it comes to the hunt it's on! Their eyes can move independent from each other which helps with hunting.
I do think biorb are gorgeous tanks. If you already have the tank I suggest doing small shrimp, like red cherry. I have a colony in a 5g and have been pleasantly surprised at their entertainment value! I love the wee critters.