i would not reccomend a 75 as a permanant home for a volitan, although they do not require the massive amount of seimming room that say, a tang or angel needs, they do produce one heck of a bio load on the tank. i would say your best bet would be an antennata, or a zebra dwarf lion, both look somewhat similar to a volitan without obtaining the massive size. my volitan lives in my 75 currently, but he is only 4 inches long in the body, once he reaches about 5-7 inches he will be moved into one of my larger tanks.
pretty much all lionfish have the cool lion doggy like personality, i would just stick with a species that doesnt get too big, and watch out for your niger getting nippy, although nigers arent known for being aggressive you have to remember he is a trigger and is capable of it.