Well, technically the code does cover these low voltage systems. When you're running them in conduit, all the same rules as the line voltage stuff apply. Yes, the grey PVC conduit and the white plumber's fittings will glue up just fine. You say that you won't have any trouble fishing this wye? I fully support a man's right to do things at his own house wrong if he wants to. What I can say is that I wouldn't do it this way. Will any one of these two conduits accomodate the necessary cabling, leaving the other as a future spare?
"Nothing is foolproof to a sufficiently talented fool"