While I would not completely discourage what your going to attempt, it will not be all that easy. You should also determined a sufficient replenishment source of flatworms should the nudi eat all the ones you have.
The >>Chelidonura varians
<< as you've already said, only eats flatworms. Depending on the flatworms/planaria that you get, they should not be of concern to the fish or mobile inverts in the tank. They can be an unsightly mass once they get going but are rarely deadly to anything including the corals they may attach to.
The main thing to worry about with planaria is if the colony crashes 'en mass, they will poison the tank much the same way a "cuke
nuke" would. They have toxins in their body that will be released when they die and if sufficient number...