I don't have a Roku or know much about it. I assume you are referring to JRiver Media Center not being listed on
this page as a tested DLNA server that the Roku works with?
Just because MC hasn't been tested by Roku doesn't mean it won't work. MC is a DLNA server, and that is all that is required. There may or may not be any issues using a MC DLNA Server with the Roku.
Try it.
PS: A Plex DLNA server running on the unRAID shouldn't interfere with a MC Server serving the same media files. Maybe there is a small possibility of file contention if both were reading the same file at the same time, but even that probably wouldn't be an issue. If either was updating tags in the files, which MC certainly can during playback if tags like "Number of Plays" were being written to the files, then there could be an issue. But you can control which tags are written to files.
Generally, it should work fine. Generally, using two media servers will reduce the benefit of having a media server. i.e. Tracking number of plays, or if a program has been watched, or where you are up to in a program (Bookmarks), will be stored on two separate servers. So just try using the MC DLNA Server.