IMO, there's nothing wrong with feeding fish to pacmen, but it's not natural though. But everything natural is not always good. In the wild pacmen mostly eat worms and other frogs, and since we know other frogs might contain parasites we don't do that for our pets.
I feed goldfish to my pacmen all the time. I also make sure that they are healthy, swimming and aleart and no side of rot or fungi on them. If you think the fish isn't being fed properly, then buy some flakes and feed them for yourself.
The best way to feed the horned frog a goldfish is to remove it from its container and drop the fish infront of it, or if the fish is larger u can hold the fish with tongs infront of the frog and it will bite at it, this wont be as stressful since u dont move him around.
i would not get the Ray Hunziker book if I were you. It has pretty pictures but in my opinion it's just some basic info and a lot of wrong info, especialy on feeding and setup.
There's plenty of websites and this message board on the web with lots and lots of info.