The way I see it is any LFS fish that he buys for that tank has a much better future than any of the others there... the op seems like a responsible fish keeper and we all know 99% of the LFS fish will be stuffed into tiny tanks with wc every 6 months or just general poor care...
That said the smaller species are a better option, I for one believe an oscar needs a 180g after owning one at 14+ inches, I have no issues with someone keeping one in a 90g though if the person maintains the tank properly.
I think a lone 7-8 fish that isn't an overly active species will be perfectly happy with proper care and decor...
Plus I'm sure in 3-4 years when the said fish is approaching max size the said responsible op will know himself if it's time for an upgrade.
As far as puffers a lurker like hairy puffer would be totally at home IMO.
This is just the way I look at it, I would think make sure the absolute possible max size is 8 inch, not a species like GT that we all know average 8 inch but freakish 12+ inchers are possible.