You can crop mothers to keep the short. I keep my mothers in dirt 5 gallon buckets. Mothers in hydro grow to quick and can take up alot of room. Dirt give them a little more of a buffer.

I have grow SOG for a very long time my self.

As for rootbound yes. In hydroponics we worry more about the root ball size then the length of feeder roots.

If you want to keep mothers in hydro then how about a 3 gallon dwc bucket system? Very easy to build and maintain one control bucket can feed many mothers as they all are in the same cycle.