Two things to be mindful of
1. like Sorin says, material characteristics should be considered, e.g. oil will stick to acrylic but NOT vice versa
2. if you enter a competition or a show they will determine the rules of what is acceptable. Transparent watercolor societies expect your work to be just that, made from transparent watercolor not mixed media. Don't like the rules, don't enter.
otherwise, feel free to mix, match or whatever, after all you're the artist and it is your art.
<insert suitably humorous or philisophical signature here>
- Eric -