You should separate him from the other fish in the 90 asap. They are not compatible based on water requirements, feeding or aggression.
The GSP is a brackish fish that will eventually require full marine. All other fish and inverts in your 90gl list are FW.
They do best on meaty, hard-shelled foods such as snails, shrimp and shellfish.
And while a GSP is no pansy they can be easily outcompeted for food in a tank with a bunch of rambunctious South and Central American cichlids. They will also quickly outgrow the GSP, set up territories and mature faster and it's very likely that the GSP will not find peace in such a set-up. Keep in mind that oscars grow VERY quickly and will eat anything they see fit to swallow or at least try. An oscar trying to eat a GSP is enough to kill both.
If you want to keep your GSp I recommend a minimum tank size of 40 gallons, some salt and a specialized diet. They are amazing and very personable fish. Very owner responsive and deserve proper care and maintenance.
Visit
The Puffer Forum for more info and feedback on how to properly set-up and care for your GSP.