The only reason your celebs and threadfins are schooling together is because they have no choice in the confines of home aquaria. They need to have schools of their own specie, but barring that they have to make do with what they have.
A neon tetra will school with glolites if it has no other choice. At least those are closely related. Celebes and threadfins are only marginally related -- they probably have a common ancestor -- and they should not be considered the same kind of fish at all.
FWIW, in Germany and other countries, Celebes are called "Celebes Sunbeamfish" not "rainbowfish" because they are not rainbowfish. It's only in North America that they are called rainbowfish.
The same holds true for all Madagascan "rainbowfish", they are not true rainbowfish and shouldn't used to form a school of rainbows.