I have read in some study, it was a while back (3-4 years ago) when I was considering beef heart, that feeding fish mammalian meat causes some type of deposits in the fishes liver or somewhere that can reduce the life expectancy of the fish. If I recall correctly it might have something to do with the fish not being able to completely break down the mammalian meat, as Cich Anon already mentioned. My guess is if a fish has it rarely like once or twice a month it shouldn't cause too much of the deposits in the liver (or wherever it was) and shorten the life of your fish but, I cold be wrong. Sorry I don't remember the name of the study or where to find it but once I read that I skipped beef heart.
For my Bichirs I feed market shrimp, talapia, kens sinking sticks (earthworm, catfish pellets, veggie sticks) soft and moist krill, NLS, Hikari Sinking gold. Had very good growth from some Endlis.