Hi, newb to your board here.
I'm freshly back into freshwater tropicals after taking about 15 years off from Dempseys.
I set up a tank with rainbow sharks and barbs. My informant states that rainbows are similar to red tails, so I thought that I'd chime in.
I find that my rainbows also don't seem to like flake food, even the small remnants left over by the sloppy barbs. They'll take it in, and prompty spit it back out. They do, however, seem to like algae tablets. Make sure to drop them out of another fish's territory or a small/new shark will often be intimidated and won't eat.
On another topic, I've been told to be careful with the amount of blood worms, at least with certain types of fish. My local store (seems to be VERY knowledgeable) states that barbs can become severely ill with large amounts of blood worms, though they'll do well with smaller amounts. Like everything else-EVERYTHING IN MODERATION.
I wish you luck with your sharks-mine are just reaching the 8 day point.