110 gallon tank for free?! Dang - I wish I could find something like that in my area. That's a lucky find! I'm quite insanely jealous.....as I'm on the prowl for a bigger tank (75-100 gals) to house a green spotted puffer or if I can find a bigger tank, a porcupine puffer. I LOVE the dogface puffers, but seeing that they require a HUGE tank, don't think that's entirely feasible (I don't think my floors could handle that type of weight).
For a 110 gallon - f-8's would be tiny (3 inches as an adult)! However, if you're sticking to brackish, f-8's would be the most logical species to put in there. If you wanted to go saltwater, I would suggest a green spotted (6 inches as an adult) as they require full marine as adults. So, as in brackish, f-8's sound good. How about a dragon goby? They're interesting species for brackish water, too.
Keep up on the updates - I'm really curious to see how it goes!