Unless you are doing super critical listening of multi channel music, I wouldn't sweat mixing speakers. Rear and surround speakers don't need to exactly match the front since those generally don't have the same sounds coming from them as the front. Much less panning from front to rear than there is from left to right. Matching is ideal, but not critical.
Do you have any spare speakers lying around to test? If not, head over to your local store with a generous return policy that has a speaker you would be interested in. Try it out. If you are happy with the way they blend with the Pioneers then keep them. If not, you can return them.