most aggressive fish get pretty big. Trying to find one that will be sustained by a 50 may be tough. I have had both of the lionfish you mentioned and I currently am loving the radiata. They can be a trick to get eating tho.
As for eels, I am a fan of the white-eyed moray. they are good eaters, but don't get much bigger than 18 inches and will be fine in a 50. I had a glass top over the tanks with eels.
Triggers will get too big for the 50 in time. The majority will decimate your corals, even mushrooms and leathers. The picasso (humu humu) is beautiful but will eat all your coral. The pink-tail and niger triggers don't eat coral. I have had both and they behaved well around coral and even cleaner shrimp. If they are small-ish you could push either of them for two years or so.
As for groupers, again, all get massive. the miniata grouper (the red one with green spots) is incredible, panther is cool as well as the blue lined grouper. The blue lined only gets about 10 inches or so and is really slow growing. I had one at the early days of my tank. It was cool and never ate a fellow tank mate. Good luck, I am excited to see pics of the livestock
-Doc
