I know of some people that have had success with Red tailed black sharks - very similar to rainbow sharks. Sadly i know a greater number of occasions where RTBS have been terrorised by Mbuna and have had to be removed.
The natural habitats and behavious of these fish are IMO too different to house them together - that said, it has been done and if you want to try it - go for it. Might be an idea to have a "plan B" for the rainbow shark just in case.