As mentioned before, the sticking to only your own kind seems to be only with certain ones. Ducky and Petrie didn't seem to exhibit any of that when they first appeared in the first movie. Ali did, but Rhett didn't when he first appeared. Shorty was startled by Petrie suddenly appearing but after getting over it I don't recall him making any sort of remark to Littlefoot like, "yuk you are friends with them, no longnecks." so I'd guess those feelings may only be in certain areas outside the Great Valley. Ruby's parents didn't seem to have any prejudice when they met Ducky or Spike. Also Mo & Elsie didn't seem to have any either. So I'd guess it may be either prevalent in certain areas, and / or, only certain herds.