If it is true that just because you can find altum and UFY in the same habitat in nature (or any other species that may coinhabit together) this does not necessarily mean they can get along in a same aquarium, we kept them together in a 250 gallon tank in the early 80's (UFY was not even classified yet I believe) and there were no issues.
I have also seen M. insignis actually schooling together (as a single group) wilth altum in nature.
Now, you have a very large tank, if you keep your number of specimens to a low density population, I don't think you will have problems. If you want a higher population, then maybe UFY may not be the best choice. I have only seen them become aggressive when they are guarding territory for spawning. Otherwise, they are peaceful.
This said, I have had schools of altum that are quite aggressive, as well as trouble making Mesonauta and other cichlids, but I would never consider UFY normally aggressive, compared to some other species. Tank size makes a difference.
God listens. He may not always give you everything you want, when you want it; but he always gives you what you really need, when you need it most!