it won´t kill the plants, but it will slow down the growth. if they must be cut, be sure to make clean cuts, and leave them as long as possible. if you are already into flowering, i wouldn´t do it. On the other hand, if they are vegging and you cut them, after they recover they will grow right back.
it seems like the best thing to do would be to either split half the plants into another reservoir or get a bigger tank.
he is absolutely right, I am telling you from experience.You can cut roots!! When you have to move them and they are all intertwined you have no choice but to pull them apart the best you can. I know this does not hurt them. Of course the more roots the better but you have to do it. And they grow right back and it won't hurt a thing. you can take off up to 1/3rd and no slow down! Have taken one that had a hair ball like a bowling ball but had to sacrifice its roots to like 3 inches and it slowed it but it came back in like two weeks and was a great success... GL