Is insulation necessary under slab with hydronic radiant heat?
If you want to do radiant infloor heating you will need to insulate under the slab. If you don't you will constantly be rejecting heat to the ground underneath the slab, and it will suck excess heat out of the system.  If your not going to insulate beneath the slab, abandon the idea of radiant infloor.

Your also going to have to use HE Pex for radiant infloor. Standard waterline Pex is not allowed for hydronic heating systems.