Ditto Brett's comments. I have a 90 w/ 6 fish, a Large Sailfin tang (had the bastard 5 years now, I can't trade 'em in), 2 Perc's, 2 Bangaii Cardinals, and a Yellow-striped Goby. I've kept many more fish than that in my 55 before, but having been able to observe different stocking levels, I feel strongly that a lightle fish-stocked tank is a much better environment (both for us and them). I also have thought about adding a couple more fish, just to make it a little more active to watch, but haven't really found anything that would fit in a good niche (besides perhaps some goby's). My Tang's too big to suffer a similar fish, and there is simply not enough room for two that dwell in the bottom half of the water column. That leaves me w/ sand dwellers, or top-water species. Try comparing what you have now, and where they spend their time, against what you hope to add, and where you think they would spend most of their time.
In my situation, I'd feel comfortable adding a pygmy angel, as it would spend much if it's time inside and around the rockwork, and right now, nothing else really "lives" in that area. Likewise, I don't have any sand-dwelling fish, so a pair of goby's would do nicely in my setup (IMO). But anything that's an open water fish that likes to swim, is going to conflict with my tang, so I know I can't get anything like that. Topwater swimmers relegate me to Anthias, a few wrasses, and Chromis basically... If you give this kind of analysis a try, I think you'll be happy w/ your decision on additional fish.
- Mac