Another example of the lack of absolutes in this hobby.
I have successfully kept angels and jumbo neons together in the past.
I attributed that, in part, to the fact that I started both of them young. So I assumed the little angels did not automatically view their tankmates as food, and the neons grew up knowing how to avoid their predator neighbor.
On another occasion, I attempted to introduce a school of neons into a tank with 3 medium-sized angels. It was pretty amazing how quickly and effectively the angels worked together, systemmatically herding and devouring every single neon within an hour or two.
Impressive, but a little expensive for feeders!
In short, with small angels, I'd assume you could enlarge an existing school of cardinals. But don't be surprised if one day you find your angels' tastebuds changed...