Hi i have the Hassio image installed https://home-assistant.io/hassio/ my question is if i can install separately the plex media server?
Thanks
Plex Media server would run like crap on a pi. It will not be able to transcode anything, and the pi has issues with certain media files. You really should run plex media server on another machine, that actually has all your media on it.
Aside from all that, there is no way to install plex DIRECTLY on a hassio image. hass.io is a docker container system. There is nothing there for you to install on. You would need to use a docker image.
If you don’t need video transcoding, Plex Media Server runs really well on a Pi - I’ve been running one since March last year, direct playing to an X-Box one and and Android tablet. By pre-encoding all the videos to something compatible it’s been absolutely fine.