IMO the red tails stubby cuda are easier to keep than yellow tailed elongated cuda (sorry forget the names now).
Joannas can be a real pita. one scare and the cuda could bash it's nose well flat. or worse if there are no lid provisions. i have one dat and it punks on a school of orin 6 pbass of same size. I've heard of them gouging out the eyes of an armatus. So prob not a good idea unless the tanks big enough for the cuda to make a dash. Much much higher success rate if you grow the cudas out prior to introducing to cohab where other fish are significantly larger. My largest cuda 1ft long isnt afraid of anything, doesnt dart around like the smaller guys.
Damn hybrid... that's what i call an experiment. now i dont feel so bad. I also buy in 6's and last couple batches end up with magic number of 2 each time. loss ratio high for me because i was impatient and introed at small size. congrats on the two. I suggest grow them out in another tank first if there are other non cuda aggressors in tank. between cuda species / size differences i observed no aggressive infighting. i must agree big cudas are really really nice.
Where are you planning on getting them from? I've found that falcatus don't hold up well to shipping so you're better off buying established fish from an LFS if you can.
In my experience, newly introduced red tails are extremely sensitive to fin rot. If existing tank mates (I'm mainly worried about the johanna) decide to take a nibble it can be hard for them to recover. I've actually had better luck with the yellowtails (I originally bought them as isalinae but the lateral stripes eventually faded and they grew pretty quickly) but ultimately lost them due to jumping.
Lightning fast hunters and very aware of their surroundings. Cool fish.