Peter it can't be said for sure whether it is male or female, but it can be said for sure that if you introduce a small one, the one you have will be the dominant male and the new little one will be the female.
I wouldn't do the same size. Get a smaller one. You don't want the anal or dorsal fins to be pointed because then it is probably transitioning to a male already. If you can find one with the eyespot still that would be ideal.
How big is the one you have now?
FWIW, most flame angels in the industry are from Christmas Island
FWIW, most flame angels in the industry are from Christmas Island, then Marshalls, then Vanuatu/Fiji/tonga. CXI has the mass collections and export all over the world from Hawaii. I can almost guarantee most in any LFS on the mainland will be from CXI. Some are nicer than others. The one from Blue Harbor is called a tiger flame, and a nice male at that!