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

Wouldn't it be possible to get trolls this way from the 6th game?

Are you talking about the snow trolls?
In general, regarding transferring models to the 5th game, you should contact Wizard Necromancer.

Added 4 minutes later

Did the hydras' tails atrophy after the cataclysm?

In Ashan, there are many hydras, some with legs, others without, some bigger, some smaller, and anything is possible! :)

Hydras are generally creatures of dark forces... how will they look in the Sanctuary? Maybe it would be better to give them to the Lizardmen?

We'll fix their faces so they smile. Hydra comes from the Greek word for "water." - this made me think of sending them to the Nagas.