I have just installed Rens and upon starting a new game (clean install for FCOM) i found that all hair vanilla and rens is missing although only on the mystic elves. Does anyone know the cause and/or solution to this?

Double check the install, i.e. that you have all the resources installed. Post your load order via Wrye Bash: right-click file-header > list mods > paste into post.

You need to install both HairOnly and Race meshes and textures, but use only Ren_BeautyPack_full.esp