I don't think Cera would ever get infatuated that easily. You'd have to find a threehorn who acted exactly like her. Then again, this could only stir up competition with her. Cera really doesn't show what her "type" is (really none of them do besides Littlefoot and Ali in a way), so it would be really hard for her to instantly fall head over heels (or in this case head over paws) for somebody.
Petrie did show quite a bit of anger when the gang was against Pterano. You also could argue that he showed great anger at Littlefoot in LBT XI. He and Ducky really let him have it about the tiny longnecks. However, to get him really ticked off? I would say have something happen to Ducky. He obviously cares a great deal about her and she him, so maybe that would produce some more steam.
Well you could say Ducky feels disgusted with Spike in LBT VIII at first when he decides to go with Tippy's herd and leave the Great Valley. Of course, this quickly turned to missing him a great deal, hence leaving to pursue him. For her to be humiliated, I think you would have to have her be arrogant, which she never is. Humiliation is more connected with Cera's personality, but since Ducky does not boast on a regular basis I couldn't really see her becoming humiliated over something. In terms of disgust, maybe if one of her friends or Spike did something utterly terrible she would show the same feelings she expressed to Littlefoot in LBT XI.
Littlefoot also feels aggravated in the TV series when Ali refuses to play with them, and he had every right to do so. If she told me that I couldn't build a mountain with my tail (or whatever she said to him), I would have said, "I bet he couldn't kill Sharptooth, or find the Great Valley without any adults b****"

. Littlefoot gets aggravated often against Cera, so I'm sure just dishing out conflicts between them would bring out more of that feeling. I actually tried to express more aggravation from him in my first
Revenge story.
I actually wouldn't think Chomper felt all too alienated in the valley. When you think about it, he's there with all his friends, his parents pretty much "hired" Ruby to take care of him, and the grown-ups accepted him in the valley (minus Threehorn). We really don't know how many adults disapprove of Chomper's presence in the valley. We at least know that Littlefoot's grandparents don't have any problem with him.
I think of Ruby the way I think of Ducky. She really doesn't have an arrogant side to her, so I doubt I would see her in a bold manner. However, you could argue that she was pretty bold in the TV series when she wanted to return to Hanging Rock alone, even so much as yelling at her friends for coming. I also would say that panic would really fit Petrie's personality better. However, the only way I would see Ruby panic is if something terrible had happened to either Chomper or her family.
Very nice topic though onto the character's feelings. It would certainly be interesting if the characters showed these emotions off a bit more, but I'm happy with them the way they came out.