I bought the season pass a few days ago, and it separates the playlists into DLC, so there is a rotation for the base game, one for Outer Rim and one for Bespin (not sure if this is how most modern FPS handle DLC or not). I'm not sure if it's a problem at my end, a problem with the matchmaking service or simply because the install base isn't big enough on PC, but I can only ever seem to find a match (occasionally) on the base game and it's always walker assault which, like you said, gets stale rather quickly.
I've only managed to find a game on the Bespin playlist once and never got to play fighter squadron (which is actually the only real reason I wanted the DLC) so I'm pretty frustrated right now.
Would love to get some sort of squad together though!