If you're not seeing any aggression between them at this point, I'd leave well enough alone. I don't have a Falco, but from what I've read since it's one of the smaller hawks it will probably do fine with your mandarin. Your idea about target feeding the girl starting with live brine to start is a good one, just be patient with her. Any potential (and I don't expect this to be major, anyway...) food competition should be offset by a few target feedings now and then.
Gemmy, in my experience the concave belly is NOT a last sign, and is actually reversible. The point of no return from what I've seen is usually the decimation of the muscle along the flanks. I've kept 4 mandarins if memory serves (and I'm stuffed with headcold medicine so this is spotty at best). My first one was lost to dosing the tank with "natural" ich medication....LETHAL to anything with an already thick slime layer BTW...., the male and female that I have now are 2 1/2 and 3 1/2 years old, and one that I rescued from a friend that I tried to resurrect.
Of my two current fish, the female had a concave belly when I transferred her from old seahorse setup into the big 110g tank. That tank was large enough that there were more than enuff pods to go around, and she bounced back very quickly. Naturally, it also helped that both mandarins learned to eat frozen in time.
The "rescue" fish came to me in very poor shape. I had her in a net breeder and tried to ply her with live brine, live tigger pods, even tisbe pods in the breeder. She was terribly thin and her flanks were heartbreaking to see, and she didn't respond to any food presented. I may have a picture of her somewhere; let me try to find it so I can show you what I mean.
