Ok

Let's see...
There's the Amazon puffer, they are very docile and need 15 gals each. They love living in groups. You could have up to a dozen in there. It'd be so cute! But it might be hard, especially for a beginning puffer-keeper, to keep their teeth down, as they have extremely fast-growing teeth, even for a puffer.
If you wanted to do dwarf puffers you could have like 30 of them. LOL
A Fahaka would love that home, and they get very large, about 18"
Brackish isn't that difficult... but if you don't want to get into marine at all then the figure 8 is pretty much your only option there. They can stay in low brackish their whole lives. And they are relatively peaceful with each other, and you could fit quite a few in there.