User blog comment:B214/Question and Answers 3/@comment-62.178.18.58-20140813224147/@comment-24502975-20140821231453

Simply put, he can marry all of them without a problem. Rias would be recognized as the official/legal wife, with the others being his other wives or "mistresses" if you wish. As for who would be the next head of the clan, that's easy. Compete for it. Same thing Sairaorg did.