1. The 5751 can be substituted for the 12AX7, but ONLY in the input position. Using it in the gain position will cause the phono pre to not operate correctly.
By "not operate correctly" I am sure he means that the RIAA EQ will fall a bit outside of specified +/- error range. It will probably sound quite fine, for the most part, and at no risk of damage anywhere. But sure, it’s best to follow the manufacturer’s / designer’s recommendations here.