Your options for a kick a** FO in a 90 are limited. All Angels and Tangs would eventually outgrow the 90, as well as most Triggers. Now if you are going to upgrade to a bigger FO in the future you can get a ton of small Angels Tangs, and Triggers, andwithin 2-3 years get them into a 200gal+.
Otherwise I would look at Eels, Lionfish, Dogface Puffer, Harlequin Tusk, and more.
On a side note, every Lionfish we sell dwarf or large, is eating prepared frozen food before we sell it, or it does not go home. Not for nothing, but currently we have 2 Volitans that eat flake food...and anything else that hits the water...
Everyone else is right on as far as the other stuff goes. Only thing you should have to check and ocaisionally dose is Alk, and PH.