I love watching my fish grow. I got all of my Koi
when they were under 6 inches. Now they're at least a foot each. My fancies have put on some size, too.
Those comets will love a 100 gal tank! Congrats on being a responsible Goldfish
owner (they are such a rare breed). One of my biggest pet peeves is when people don't do their research on the fish and then mistreat them.
Distracting the goldies with food sounds like a great idea to feed the bichirs. An orange might be enough to distract them. You could also try feeding the bichirs after dark. Goldfish
are more of a sight-feeder, and if they can't see the food, they can't chase it. Another idea would be to use your hand to put the bichir food at the bottom, maybe under your cave.
I keep the orange slice in there no longer than a day. I usually put them in around noon and take them out before lights out. I don't like keeping it in there overnight because while the fish sleep, the little bits of orange still on the rind tend to fall off and get caught in my filter. It might take your guys a few times to figure out what to do with it, but when they get the hang of it your orange slice will be gone in a few hours.
As for food for the bichirs, I couldn't tell you. The largest truly predatory fish I own are my bettas. Small mouths are pretty simple to feed (lol). But I think the people over in the "Ancient Fish" subforum can help you.
You might not have to totally skip feeding your goldies. You could try only offering them greens for a week. They would still be eating; they just wouldn't be getting a lot of protein and will hopefully slim down. Time to put them on a weight-loss diet!