I've kept these fish off and on for 15 years or so, and I can tell you that an emperor angel of 12" in captivity is possible, but rare. 16" even more so. My current one lives in a 150 gallon reef. He's grown from 2" or so to maybe 4" in 11 months. I don't feed him as much as I could, but he get's enough to stay fat and healthy, and show some growth. I don't WANT him growing too fast for several reasons. First, I love the juvenile color pattern, and second the smaller he is , the better. If you feed heavily, they still tend to get to 7" or so, then slow way down. I've done a lot of experimenting with angels to detrmine what feeding regime is really necessary to keep them healthy. While intentionaly stunting one is not necessary, nor is it necessary to feed them so much that you get the maximum growth rate. This is captivity, and one should keep in mind that the metabolism of many organisms adapts itself to the feeding regime. Angels are no exception here.
Also, many times the change into the adult pattern never completes itself in captive fish. This problem could be dietary in nature, but we really haven't pinned it down yet. In any case, er on the side of the fish and feed as varied a diet as possible. Keep in mind that as youngsters, they are cleaners and take in a fair amount of crustacean foods. Their diet becomes a bit more eclectic as they mature, and high quaility angel formulas are your best bet. Emperors are not obligate sponge feeders as are some angels, notably some of the holocanthids - so don't hesitate to offer meaty foods as well.
Getting them to eat can be problematic, and starting them on live brine is sometimes the only way to keep them going for a time. My current imperator would only eat live brine for 6 weeks or so. Then he started on frozen. Now, he eats anything I put in there including flake food. An individual here and there will not acclimate to prepared foods in time to avoid a severe loss of weight, and the accompanying lapse of the immune system. A risk you take, but you can minimize this by ordering a tank raised specimen.
These fish are among the hardier of the pomocanthids, but this is a realitive term here. Cyanide poisoning is a problem with this species, and a juvenile from Christmas island is your best bet. This last time around I took a chance on another locality - twice. The first time he did a cyanide crash within 3 days. The next one did much better and is still with me a year later.
Best of luck
Jim