I think most of those triggers will outgrow that tank, and he said he didn't want to upgrade. I think you could keep an undulated trigger in there for life, but that would pretty much be it, not only due to its size, but its temperament.
When you say you want an "aggressive" tank, what do you mean? Do you have a centerpiece fish you really want? Most people would consider your current tank to be aggressive
(with the "leaf fish, snowflake moray, yellow tang, bristle tooth tang, molly miller, soldier fish, tomato clown and clarki clown".)
Some fish that I consider somewhat aggressive that you could keep in the 75 include:
wolf eel blenny (one of my favorite fish for unique, odd looks and engaging personality)
dwarf lionfish, especially the fuzzy dwarf
sharpnose puffers (tobies) - they can be fin nippers, but will not outgrow your tank and come in a variety of nice colors and patterns
dwarf angels
waspfish - you already have leaf fish, which I love. I have longspine waspfish too. they are a little more active, and have interesting behaviors. there are also many oddball small scorpionfish you could keep, but you have to look for them.
angler - peaceful in a way, but a fish and shrimp eating machine. anglers are the coolest fish around, IMO. you could get a commersonii, which gets football sized, and make it a species tank.
toadfish - I have an orange toadfish in a tank by itself at work. it is a voracious predator, but you can keep it with other fish as long as they don't fit in its mouth. I had mine with my waspfish for a while and they didn't have problems. the toadfish will swallow live fiddler crabs whole.
you couldn't mix all these fish, there are definitely compatibility issues to consider. but these are some smaller predators to think about.
