i do not know if subscription is the only way or just pay for the app.
I have plex server on my Minix z64 (low pow pc) , i bought the windows app since i had some money to spend from xbox live, so i can play files from my pc using the plex app.
On my smartphones, i got the app for free from amazon, they put it quite often on their free apps, i have it on the devices i use for my daughters, a tablet and a pair of smartphones, since it is technically a 'bought' app, it can play what is on my server on those devices (two smartphones and one tablet).
On my wife's smartphone and mine i have the app from Playstore which won't let me play on media on them but allows me to send to the chromecast on my tv.
I also play my media from my ps3 on another tv but instead of using the plex app (paid app) i just browse through content as a media server , and works just fine (it is not as nice as navigating with the app, but gets the job done)
i live in spain, nexus 6 was the first nexus to hit carriers, so my guess is 6P will have the same treatment, and 5x won't be supported by them. But i guess we will know in a few days.