I don’t know if his vocabulary indicates his level of knowledge either way, but he can be trusting and innocent, qualities he does share with Chomper. I do see the believer point. The Bright Circle Celebration does show he’s the kind of person who celebrates a holiday because he does believe the story around it (Whereas Mr. Threehorn disbelieves, and Tria just think it’s a good opportunity for family and community time). I didn’t find all of Petrie’s fears realistic ones either – the various times he believes in bad luck, among other incidents (Alright, the belief in bad luck mightn’t be too farfetched in-universe ;]).
I didn’t admittedly consider Chomper paranoid fears in the TV series to be farfetched to his character in his two movie appearances. I guess I surmised he might have developed some fears as he grew older (Well, grow older by kids cartoon show standards) as many of us do and I kind of felt we only saw a small slice of his life in movie 5.
As for movie 14, the extent of Chomper and Petrie’s interactions were at the end, where Petrie asks why he and Ruby came, Chomper said they came to rescue the gang, and Petrie thanks them but proudly states that instead they do rescuing (of Bron). Chomper and Ruby are surprisingly chill about not being asked to come along. (I watch the movie on US Netflix. As for the TV series…there are other ways. >.>)
Moving onto other interactions…one I’d be interested in seeing more of is that between Cera and Hyp. I know in Hyp’s two appearances, he was often paired up with Littlefoot, but I was intrigued by the blinks of interaction between Hyp and Cera. When Cera demands he give their ball back in Hyp’s posse’s first appearance, she growls at him and he growls back. In some ways, they are foils to each other, which is made explicit with their fathers. Cera didn’t have much fondness for Hyp and his friends for understandable reasons but as the latter improve themselves, I can see her warming to him slightly.
Though the pair might still find each other annoying. Even if Hyp moves away from being a bully, he’d likely still get into trouble and be a wise-butt, and that’d annoy Cera. Hyp might find Cera a pain because she can match (or maybe even surpass him) in the snark territory and wouldn’t hesitate to point out anything weird or absurd they might be doing. Nevertheless, I can see the pair coming to enjoy the battle of snark and insults that happens when they are in hailing range and that becomes how they express a sort-of affection for each other. When either of them need assistance, especially with family matters, they could either be blunt and direct in telling the other to get ahold of themselves or more gently suggest how to pull themselves out of a problem.
So basically, they could have a scrappy yet affectionate sort of friendship. I can picture Cera saying “He’s like the big brother I’m glad I never had” and Hyp clapping a hand on her before saying, “She’s like the little sister that makes me glad my parents stopped at me.” Then she’d throw his hand off. An amusing and often roughhousingy kind of friendship. When Cera is bigger than him and he’s being annoying, I can see her (gently) stepping on top of him, either saying some variation of “Did you hear something” or “Pardon him, he needs a bit of rough treatment to shut his mouth.” Complete with her occasionally turning up the pressure to elicit some amusing “oophs” out of him. And Hyp might retaliate or induce being stepped by climbing over her and using her horns as a resting place. They would be careful not to really hurt each other but they’d annoy and rough one another up as one way to express affection and it’d be all in good fun.