A niger and a huma huma would work fine in your 75gal, that is if they get along (which they should). However, you would want to steer clear of putting a lion in with them. Whenever I've seen lions with triggers the lion seems to not appreciate the company. Lions are more docile and like to relax, well except for feeding time. Where as your triggers (especially the huma huma) like to swim back and forth all day. This usually annoys the lion and he nips at them. Of course causing no harm, just a get away strike. The lion couldn't really do anything obviously with a bite.
Anyways, the triggs would be fine in a 75 together and the lion I wouldn't put with them cuz of difference in personalities. I would put an eel in with them though. A snowflake would work fine, although I now stay away after my snowflake got out of a nearly unescapable tank. You can't over seal a tank when one of them are in there. I wouldn't put a tang in for obvious reasons like: they are prone to ick when stressed, and a trigger can easily cause that tang some stress.
P.S. The weird thing is, the night my eel got out there were 3 other members who had theres escape. I think it had something to do with the moon but who knows. Anyways, it was strange.........and NO, a 75 gal is not too small for an agressive tank. Just don't put in too many or too large of fish. If you would have said your putting in a large trigger like a clown then I would have said don't do it. But the ones you have picked stay relatively small and will not outgrow the tank if you don't add other fish and keep it that way. That is the trick. HTH and take care. Just my 2cents (which is worth a whole lot to me=)
