Not all the dinosaurs were as strict on the "keep your own species" thing as others like Mr. Threehorn was. Two other examples of dinosaurs who didn't abide completely by this belief include:
Rooter (He didn't have to talk to Littlefoot. He didn't have to try to cheer the boy up. Yet he does, even though Littlefoot wasn't of his species.)
The Baby Pteranodon (Even though Littlefoot was another species, the baby pteranodon still felt sorry for Littlefoot and still tried to help him, willing to sacrifice its only bit of food for him)
I agree that Ducky's mother is probably the most extreme exampl of acceptance in the first movie. As evidenced in movie 8, she took in Spike because there weren't any Spiketails in the Great Valley. However, this behavior is not new to nature. There are cases where an animal of a certain species will adopt an animal or animals of a different species. Such cases include a cat adopting a litter of puppies, a cat adopting a bunny, and a lioness adopting a type of antelope.
So I wouldn't be surprised if dinosaurs sometimes did take in species that weren't of their own kind. Probably a rare event, yes, but not behind reasonability, I think.