I had success forming a harem of multicolor angels. I ordered 3 small ones and 1 medium one. I divided a 55gal QT tank into four compartments with eggcrate. After 5 days or so, I removed the dividers and watched for 30 min as a couple of them went to battle. I decided to divide them up again. After another week or so, I went down to check on them and all of them managed to find their way into the same compartment. I removed the dividers, and they have been fine since. They're currently in a 75gal tank.
I have tried to form a harem of flame angels a couple of times, and have been unsuccessful. I think it is mostly due to ordering all small flame angels. For whatever reason, they don't seem to ship well and I end up losing them. I only have one lone survivor, and I'm thinking about trying to pair it up with one that a friend has which is significantly larger. I'll use the same method with eggcrate in a 20H.
You should be able to get a med/large size one and a smaller one. Divide the tank. If you use eggcrate, you can cut a few openings in it so that only the smaller one can pass through. That way it can enter the larger one's side and then escape if need be. The small ones will fit through the smallest openings between the edge of the eggcrate and the glass too. I would keep them divided until you see the smaller one freely go into the larger one's side and remain there and/or not get harassed. At that point, you may have successfully formed a pair (granted they prefer harems though). I do not know at what size they change sex, so there's always a chance the smaller one you got is already a male. Maybe someone else will chime in here with that info.
