Winsor and Newtons site has an article about mixing acrylic mediums with regular gouache. Which it states you can do.
I have turner acrylic gouache and have used it with Liquitexes pallette wetting spray and Jo Sanja’s flow medium, witch is designed for acrylic gouache, with no adverse reactions. My understanding is that acrylic gouache, is really acrylic paint with a higher pigment load to obtain matteness, therefor it should be compatible with all acrylic mediums or additives. Acrylic gouache uses the word gouache really to describe a characteristic of the paint (matteness), as opposed to working properties. It behaves like acrylics not like traditional gouache, at least from my limited experience. I would assume that if mixed together that the polymer resins in the acrylic gouache would try to bond with the pigment particles in the regular gouache, and I guess depending on how much regular gouache and/or water added that it all would in essence turn into acrylic gouache? I have never tried this, hopefully someone with more experience will chime in, as I am very interested myself to learn how the paint will behave.
I think they came out great. I especially like the one of the right.