Connecting amps to preamp

I don't believe you will be able to control the volume individually making this not a good idea. You want the rears at a lower level than the fronts in a surround set-up. Which gives you the out put you need for the sub. You could also run an output to the sub and high pass it to a pair of speakers (depending on what kind of sub)

I have Gain controls on the rear amp. The sub doen't have any outputs. should I use a rca Y cable?

Yes- a Y cable will work