Dark waxing over polyurethane, how to do it?

You don't need to, unless you apply the clear before the dark to make the darker easier to work with (I usually do that, but it is optional). Your wax will dry and cure and the poly will do the job of protecting the paint.

Good luck!

also have read that you shouldnt add any kind of poly to any wax or glaze

Yes I have many pieces that I have used chalk paint and clear wax on that are very durable. Nightstands, dressers, headboards, stools, benches, coffee tables. I usually put 2-3 thin coats of clear wax on and buff in between with shop towels (from a box). I have found old white cotton dish towels, white t shirt and shop towels to be the best for buffing. You want to put thin layers of wax on. It is much easier to buff to get it to shine. If you put on too much wax it's hard to buff. You want to keep buffing until your wax is shiny and smooth. You may want to practice on a scrap piece of wood to get the "feel". On your projects do small areas at a time--do not wax the whole piece if it is a large piece and then start buffing. I have not had any of my pieces yellow and I have not had to rewax yet. I use Annie Sloan wax for all of my projects. I like this wax the best.