Hello,
Lionfish? cool, plenty of room. Is a great fish for a 125.
Goldtail Eel? Also a nice addition to a 125.
School of henchious(sp?) butterfly fish? I don't think so. That would be a nice mix of color but, you won't be able to get butterflies to school in a 125. And, the competition will eventually get pretty harsh. One of those would be a really nice addition to the two mentioned above, assuming that you provide the proper diet.
My suggestion (or 2 cents), get the eel, the lionfish (I've got a 125 with about a 15in. lionfish (way cool!)), the butterfly, and like, some kind of tang that really catches your eye. Or, maybe even another lionfish. I want to add another lion to my tank some day. Probably won't be until I build that dream tank i've always envisioned though. Good luck, just my thoughts, though.