Can i add a sub to my set?

Yes.
 

You have two rooms. You can add a Sub or Sub Mini to either of those rooms, or to both.
 

But to be clear: if you want the extra bass in both rooms, two rooms means two Subs. You can’t share a Sub across rooms. 

Yes that's fine, looking to add it to living room.

Does it have to be a certain gen sub?

Are you running the old S1 or S2 version of the app? 

https://support.sonos.com/en-us/article/sonos-app-version-compatibility?language=en_US
 

Sub gen 3 is S2 only. 

Only S1 works with my setup

Why? Playbar and Play:1 are both S2 compatible- though I wouldn’t recommend switching for a few weeks, with all the fun and games we’re having with it at the moment. 

Thanks for the help, I did update the app but have downgraded it again to 16.1

However it's now saying this on play5 

The original data you gave did not show the gen1 Play:5, which is indeed S1 only. 
So, stay on S1 and get the Sub compatible with that, as answered above. 

The issue now is the below on S1