+1 for the Marine Betta. Must have fish for my own tank and it’s going to be among the first fish I’m putting in, with the other two being my Ocellaris pair.
Lionfish, the smaller ones that get up to 8” will do. Make sure you can feed it though, it’s best to find a specimen at the LFS that’s taking at least some prepared (non-live) foods. I’m not too familiar with Lionfish but their health will deteriorate if you cannot ensure they’re eating well and eating a steady diet.
Small Tangs-Yellow Tang, Tomini, White Tail Bristletooth, Kole Eye Yellow, Square Tail Bristletooth, MAYBE a Purple Tang (Zebrasoma genus like the Yellow Tang but gets up to 10” while the YT gets up to 8”)
Foxface-Foxface Lo, One Spot Foxface, Blue Spotted Foxface
Dwarf Angels-Any of them (genus Centropyge) will do. Maybe even a smaller medium-sized angel like a Xanthurus Cream, Bellus? (Not sure)
What about a Hawkfish or two? The Flame, Longnose, Falco, and Coral / Spotted are my favorites. Some of the other ones get too aggressive. The Flame is by far the most peaceful, I think.