What are the rulings regarding the Poe leader and the Millenium Falcon/R2-D2?

I've heard conflicting things.

So, can Poe use his leader ability to attach to a Falcon that already has a pilot/a vehicle that has R2-D2 attached, and can Poe use his upgrade ability to attach to the Falcon that already has a pilot/a vehicle that has R2-D2 attached?