Thanks for the comments guys. I've got some info from a thread on another forum that is worth sharing. There are flexible cements available for using spa flex on pressure situations. For drains and returns, regular pvc cements are fine since even if they fail, they only drain the sump (and possible ATO) For my application, I've got a manifold for a closed loop under 350 gal reef tanks that are being installed. Failure of a closed loop is a much more substantial issue!
Two products were highly recommended : Christy's red hot blue glue (as well as other "blue glues") and Oatey flexible clear cement. The reasoning is that they melt the pvc's faster and get a better bond between the materials, while remaining pliable after curing. Spa flex is pvc without the hardener added so they are nearly identical materials. Due to the flexible nature of the spa flex, you need to use a flexible cement. A hardened cement has the potential to break off its bond. Considering that there are going to be hammerheads pumping the water through these, take no chances. I"ll also use ball valves at the transitions between the differing materials - just to be a little extra cautious.
