Most hollies are dioecious, that is you usually need a male and female to cross pollinate.

Now, some species can cross with other species. Some cannot.

When you get into the hybrids, like the so-called blue hollies, they can certainly cross-pollinate with species that made up the hybrid as long as they are blooming at the same time. Again, you need a male and female plant.

The Blue hollies were introduced into the market as a 'matching pair' for this reason.

China Girl / China Boy

Blue Prince / Blue Princess

etc.

I bought 2 blue princess hollies last year not knowing there are male/female plants. this year I found some china girls & a dual china which has both sexes in the same plant. I've not found any princess varieties so far so I'm wondering whether the china holly might pollinate the princess hollies. i'll continue looking for a blue prince.

Yes, very likely, but only if they bloom at the same period of time.