There isnt one established set of beliefs in Paganism but there are smaller subsets called Traditions. sort of like if someone said they were Christian but not a member of any recognized denomination like Baptist.
This page is a great place to learn a little more about the different Traditions in Paganism and Wicca
But to go back to your original question...yes but they are generally not that radical about it. when choosing to work with gods and goddesses from different pantheons people generally (and hopefully) take care to make sure that the personality traits and energies of the gods they are choosing are compatible.
In a coven setting the members generally work with an agreed upon group of deity.
