Which grease should be used between plastic and bearings?
It is my belief that this fear is overstated, and I'd simply use Park polylube. (I've seen threads where boat mechanics discuss using their favorite marine grease on all their bike parts with no ill effects.) If worried and wanting to spend more, I'd use suspension grease, which is by definition going on plastic/rubber seals. A couple of the more popular name-brand suspension greases are just Slickoleum, re-packaged and marked up 3-4x in price. You can get a tube of slickoleum for around $12, or a bigger tub to last your whole life for around $20.