If the rule is that Republicans shouldn’t appoint Democrats, then McReynolds can’t be an exception to the rule, since he was appointed by a Democrat.
BTW, Butler’s votes and opinions were much better than McReynolds’s—really, Butler was ahead of his time on many issues. Butler was a bit weak on enforcing the Equal Protection Clause against clear racial discrimination by states, which made him a man of his times at least on that topic, but he wasn’t a virulent racist and anti-Semite like McReynolds.
Well, those attributes at least made McReynolds a much better DEMOCRAT than Butler. ;-)
My example with Stephen J. Field fits the "exception to the rule" premise though. He was a Democrat justice appointed by a Republican president. I would say his voting record and judicial philosophy was comparable to Scalia and Butler, though the three of them came from radically different eras in U.S. history.