are you saying you want the lionfish and another predator(s) in the 75? If you have a volitan lionfish in your 100 gallon reef, I'm not sure a 75 will be large enough for it. Even if the 75 is fine for the lion, you'll have to consider compatibility.
A few suggestions, which are all fish I have kept, or am keeping:
Devilfish scorpion (Inimicus spp.)
Brazilian dragon moray (Muraena retifera)
Green wolf eel blenny (Congrogadus subducens)
These are some fish I consider borderline for a 75 (larger tank better):
Dog face puffer (Arothron nigropunctatus) - consider rare color morphs
Harlequin tuskfish
IMO most fish like most anglers and toadfish will do better in a smaller species tank (20, 29 or 30 gallons). Unless you keep a Commerson's angler (which get football sized) most anglers in the trade get around 4-7 inches as adults and can be kept with very few other fish. Toadfish are also very cool, but best in a 29 or 30 gallon tabk, by themselves. I had an orange toadfish for years in a 20L in my office at work. Lots of personality, but I think it would either hide from or attack tankmates...