You're going to have to make careful choices, since the more commonly kept species in the groups you like would outgrow your tank. You *can* have a down-sized aggressive tank, if you make the right species choices, and as with any tank, consider compatibility.
Puffers - as suggested, tobies are the best choice. I have kept some tobies with lionfish, all successfully, but they have a reputation as fin nippers. You have to be prepared for that possibility and proceed with caution. I really like the Hawaiian spotted toby, very nice fish.
Eels - most get too big. I wouldn't even get a snowflake - IMO it would be out of scale with your other "downsized" predators and would also limit how many additional fish you could keep (due to bioload). I had a green wolf eel in a 55 for many years, and it was a great "eel-like" predator (it is actually a mutated looking dottyback). It also didn't bother my other fish (though they have a rep for eating whatever they can swallow, like many "true" eels, so don't keep it with fish a lot smaller, stick feed, and avoid live food). You could try a golden dwarf moray, but they are quite pricey. And I think the wolf eel is way cool. You don't see that many of them.
Triggers - there is no trigger that stays small enough for your tank. Consider smaller filefish. They have a lot of the trigger look and personality, without the size, and often without the aggression. Consider finding a pair (don't try to make a pair, it won't work) of saddle puffer mimic files. While they look a lot like a saddle puffer, they are also reminiscent of many of the larger triggers. Very very nice fish! The aiptasia eating filefish (bristle tail? I forget genus and species) are also very nice, and it is pretty common to find them offered as pairs on Diver's Den.
Tangs - there really aren't any tangs small enough for your tank. Would you consider a dwarf angel? I like coral beauties for color, personality, hardiness and beauty. Or consider a basslet (which doesn't look anything like a tang, but they are nice fish).
Lion fish - there are several really nice dwarf lionfish to choose from. Fuzzy dwarf would be your hardiest, least expensive choice. If you take your time selecting one, you can find really nice coloration and pattern. A zebra dwarf looks more like a volitans.
With the exception of the wolf eel, these other fishes generally range from 4-6" in captivity. I'd make sure you have a good filtration system, do a bit more research, and then narrow your list to no more than 5-6 fish.
Good luck.