I find that acrylic is better to use if you want to create layers with the matt medium.  It creates a protective barrier and changes the paper from porous to non-porous, meaning that inks won’t “bleeed” into the surface of the paper, and you can work on top if it like fixitive @outlander ...... but painting matt medium over watercolour will cause it to mess up ...... I have heard that PVA glue works the same, if you paint it over the whole lot, and it dries clear, but I haven’t used it like that.

If you don’t want to sew the work, then watercolours are fine too.