Some people have trouble mixing species of puffers, too, if they have the same general body shape. For example, a dogface and a porcupine might fight (or might not, but to me it's not worth the risk), but you could probably have one of the sharpnosed puffers (aka the hawaiian puffers aka tobies) in with either one of the aforementioned species.
And yeah, your bioload is pretty hefty as-is, though you COULD potentially overstock, IF you had the equipment to handle it. What kind of filtration are you using?
I agree with DrNecropolis: of the fish you mentioned, the only one I would even consider adding would be the Niger Trigger. Humus are fantastic little dudes, but they're more aggressive than Nigers, and I'd only keep it with fish of a similar size in a tank stocked like that. Sailfin Tangs will usually fight with other tangs that have the same body shape, like the Yellow Tang, and when tangs fight they can inflict some nasty damage on each other.
If you're going to add another fish, I'd go with a wrasse, personally. They're pretty tough critters who should be able to hold their own with what you've already got, without actually picking new fights...and they've got boatloads of personality.
