hiya ! Been keeping rainbows and learning a ton
Rainbows in general have large mouths but small throats. Expanding food (some freeze dried stuff ) can actually choke then. I could see a rainbow getting a cardinal part in but not be able to ..eat it… now why/if/would have no idea - I keep Bosemani, zig zag, and a couple others none have ever tried to snack on anything except given food or maybe a moss thread they think is food.
6-7 Bosemani I’d say 55 gallon or bigger and you said it was a big tank - I’d go as big a school of cards as you can. 20?..My rainbows ignore pretty much everything else in my tanks, they are fixed on each other. They are pretty active and the males will spar non stop at times, mating is initiated with a chase so … big school better for the tetras.
for the rainbows I’d recommend male/female 3/4 ratio at most, 2/5 much better male to female. If you only do six 2/4 is a must ratio to me or stressed out sick girls.
good luck pics of tank ? Love to see!
here is a quick view of some of mine