If some chicks have white barring and some do not:
--the rooster's color is still called "barred"
--the rooster is not pure for barring, because he carries one gene for barring and one for not-barred
--the rooster is not a "Barred Plymouth Rock" (also called "Barred Rock")
--yes, the rooster is probably some kind of a mix
He is certainly not the breed called "Dominique," because he has the wrong comb type and a few other details are also wrong. But the guy might be using "dominiker" to mean any chicken with black & white barring. Or the guy might be mixed up