In terms of style the original movie and the sequely (every one of them) are almost two entirely different things.
The very inclusion of Chomper in LBT 2 meant an extreme breach with the entire setting of the original movie. What I always regretted about the inclusion of Chomper is that the movie makers never, ever
really bothered to acknowledge the dilemas this means, especially for Littlefoot. It always bucked me how Littlefoot's mother was barely mentioned at all even though memory of her would probably make accepting Chomper even more tricky for Littlefoot than for Cera. They could have gotten a lot more out of the plots with Chomper in LBT 2 and 5.
Moreover there is almost nothing we learned about Chomper in the movies. It was only in the TV series that we got to know a little more more about him while based on movies 2 and 5 only it would prove quite a challenge to say much about his character beyond some very, very general traits.
The question whether or not a character "deserves to be in the gang" is also kind of tricky, for what exactly do we mean to decide when answering yes or no? Are we to decide whether the characters Littlefoot, Cera, Ducky, Petrie and Spike are permitted to have other friends even if they didn't accompany them on their legendary journey during the first movie? Are they too exclusive a club as to allow anyone else in? Or is it more that we don't object to them having other friends, but please outside the screen times?
Have the plots of the TV series suffered from the inclusion of Ruby and Comper compared to the plots of the TV sequels? Opinions vary there, but personally I think the plots of the series were not bad at all and oftentimes Ruby proved a sensible addition.
Chomper's main characteristic (at least until the TV series gave us a little more insight on him) was his species. In case of Ruby however she did in my opinion cover some characteristics which I don't think were covered by any of the existing characters (I posted my view on her in the
thread about Ruby). In any case I think she had more to offer for the plots than just the random throwing in of a new name which is so common a trick to draw attention to older franchises that it is sometimes even parodied.
Interestingly enough (looking through fanfictions) many people would probably welcome Ali as a permanent member of the group though when it comes to characteristic features she too comes mainly with shyness (nothing new among the Gang) and the perception of the "sandbox love" between her and Littlefoot that seems to make people more accepting about her than about Ruby.
My main concern about a permanent inclusion of Chomper and Ruby had been that with too many characters the focus would be on a few of them with the others being mostly ignored. However, I think when it comes to the distribution of spotlight the TV series did a pretty good job and compared to many sequels I also think that they got somewhat away from the extreme focus on Littlefoot.
Perhaps a main reason for the high degree of scepticism about Ruby may be that we have hardly any idea where, how and why she entered the story. We get a few basic statements (apparently she promised to Chomper's parents to take care of him) but no real explanation whatsoever. Rather than as a gaping plothole I see this however as an opportunity. An opportunity for us to come up with our own ideas and stories about where, when, how and why Ruby entered the stage. Many fanfiction writers say that it was impossible to write fanfictions based strictly on the land before time universe (short of humans, aliens, timetravel, warfare, zombies etc.) for lack of stories to be told that were more than repetitions of what we have had before. Personally I disagree and see a big story there begging to be told
