Yes, you can connect the +/-15V on the rodeostat to the +/-12V on the multiplexer.
Earlier revisions of the Rodeostat used a +/-15V DC-DC converter to provide the power rails for the op amps on the Rodeostat. Whereas, more recent revisions us a +/-12V DC-DC converter. The multiplexer is labeled to match the more recent versions of the Rodeostat. However, the multiplexer will operate fine on +/-15V.