Putting in new bathroom.. in one wall that is torn out we have copper lines to be re routed. Can we tie in there with Pex and then re pour over this joinder?

I wouldn't put ANY pressure joint under a slab, particularly a PEX compression. They just built a slab house next to me, all PEX with solid runs, no joints

Brian's bang on! You install Pex like we used to install soft copper. With no joints under the slab and brought up in the walls to form a manifold. Good luck, Tom