Glad to see you back, it's been a while
I would say the best work organization we can have is to have each artist dedicated to specific geographic areas, to ensure graphical consistency at a local scale. With that in mind, my recommendations would be for you to keep to the Mideel scenes and the sector 7 slums for now.
By the way, regarding displacement: if you're running out of RAM, my take would be to decrease the number of subsurf divisions, but preserve a high level of detail with bumpmapping. That's what I used for the cliffs of eals_1 scene (Aerith's house outdoors), they have "only" 4 levels of subsurf if I recall correctly. I think you can strike a good balance with a moderate poly count.