Moke's are relatively shy, but are still rather predatory IME. Syno'll probably be fine, but I wouldn't put it past one to target the vampire shrimps, especially at night.
I know this has been quite a while but I have not been able to find much information on mokelembembe care in community tanks, so I was wondering if you did end up getting one? I too have some concerns over whether it would attack my existing fish like Synodontis nigriventris and medium size (6-7cm Cameroon/vampire shrimps).