You can certainly add more chromis - if you can get them to survive, chromis are _much_ nicer to look at in groups.
You have a black trigger - is that a niger, or a durgeon? I've seen tanks where multiple triggers get along, and in six feet, with a mellower trigger like either of the "blacks", a younger huma-huma might just be OK - for a while, anyway. Shrimp, crabs and other crustaceans will be at risk - but you knew that. They'd be at risk with a snowflake eel, as well. (Do keep in mind that these big predatory fish take up a lot of your biofilter's capacity to process waste - and believe it or not, can start to look cramped, even in a six foot tank!)
You've got your eye on some big tangs who really love to move. I'd probably set the naso and regal blue (hippo) into my "someday" bin, until I had a bigger tank to place them in. (I have a 220, and am not considering those fish. Well, OK - I _briefly_ considered a hippo, but only because I have a formerly-Nemo-obsessed granddaughter.) Yellow tang ought to be good, or a purple tang _might_. Had you looked at any of the smaller bristletooth tangs? I've got a kole, and in good health, she's nothing like the ones you see in your LFS. Tomini or white-tail would be happy there too, as might a mimic / chocolate tang, which I happen to think both fascinating and elegant.
One yellow (Zebrasoma), Kole, tomini or white tail (Ctenochaetus) and a mimic (Acanthurus) ought to get along well enough in that box. Choose one from each genus to minimize aggression.
Flame angel ought to be OK - will you have coral in the tank? My own flame is pretty well behaved, but the risk exists.
As you get closer to "fully stocked", test your nutrient levels frequently. Your tank will tell you when its biofilter is handling all the current residents, and ready for a new one.
~Bruce
