RTF is a global modification that replaces all towns in the game.

then you can do it like this:
Humans - Angel and Dragon
Elementals - Phoenix and Mystic Eagle
Necropolis - Fallen Angel and Bone Dragon
Nagaland - Naga General and Kirin
Might and Magic - Monsters and ?
Dark Elves - Golems and Giants
Lizardmen - Hydra and Great Basilisk
Mountain Orcs - Stone Giants and Cyclops

I would change it a bit.
Humans - Angel (from the 6th level), Dragon
Elementals - Phoenixes, Mystic Eagle - I really like the Mystic Eagle, but then there will be 2 identical units! In general, you can put Djinns on the 6th level (or on the super-elite) (you can take Time Spirits from Nexus). And on the 7th level, Magic/Void Elementals.
Necropolis - Fallen Angel and Death Knight
Nagaland - Naga General and Kirin +
Might and Magic - Monsters and ? - we'll have to think about this.
Dark Elves - Golems and Giants +
Lizardmen - Hydra and Great Basilisk - the Hydra should either be very different from the original, or it would be better to put these "birds" based on Wyverns.
Mountain Orcs - Stone Giants and Cyclops - it seems to repeat the Dark Elves.

and at the same time, we will have the opportunity to make a third unit for each faction or seven neutrals.

What do you mean by this?