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I would say that that the limitation is not the size of the tank as much as it is that angels don't tend to like each other, especially when it's the same species.
Mixing angels is always something of a risk, but it can be done. Usually you are better off if they are different species, and one is a bit larger than the other. This way a pecking order might get established.
Angles in a reef are again something of a risk. You have some that are fine with corals, others, of the same species, go after corals, and others, still of the same species, start out good with corals, and then turn bad.
I'd say give the angels a try, but be prepared to remove them, if they start in on your corals.
My understanding is that ornate angels are planktovores and eat from the water column...in this regard they are different than most angels. I do not have any other angels in the tank. They aren't that common, but are occasionally available. The male and female look much different.
I believe you are referring to the Genicanthus Bellus angelfish. From all I've seen, they are safe to add to a reef. Like any fish, they should be well fed to keep them from potentially nipping at Corals/LR. There should also be a lot of hiding places in the tank as it may take some time for the male and female to get used to one another.
I have a pair of bellus in my 180 (already a happy pair, purchased from AO). I would think that a lightly stocked 120 would be fine for them as well. Mine love to weave in and out of the rockwork.
I myself have a wantanabe from A.O. and she's really beautiful .. one of my favorite fish. She's very tough... she lived through a week in my hospital tank while she was treated for fungus.