Nomads is worth getting. I am not sure Crossroads is... adds to many pieces and an extra scoing system which is unnecessary, in my opinion.
That was my opinion before I got Crossroads, but I like Crossroads pretty well. The extra pieces are location-dependent, and actually a more delicate adjustment than the wall stones in Nomads.
Nomads is, in any case, the only expansion to add to the player count.
To add a 6th player on top of Nomads' 5th would take more than 40 green houses, I think. I'd say use 6 board pieces.