First of all, I love that poster. Great work!

1. Considering the gang's exploits in defeating or outsmarting sharpteeth, including the original film sharptooth, what kind of reputation do you think they have in the Mysterious Beyond?
Hmmm... is this actually a case of the gang being that clever? Or is it more a case of the sharpteeth being less than clever?

But in all seriousness, I don't think the gang would have a big reputation in the Mysterious Beyond, if they would have any reputation at all. They're definitely
competent enough against predators to have earned such a reputation. But their lush society in the Great Valley versus the harsh wilderness of the Mysterious Beyond are separate enough that anyone within the Great Valley likely wouldn't have much of a reputation outside of it.
That, and the various predators they've encountered mostly seem like isolated incidents. As in, the different sharpteeth
(the ones who managed to survive, anyway) probably wouldn't all meet up with each other and swap stories and be like,
"Oh, did you hear about those kids? Yeah, they're a nasty bunch. They look like they're harmless and easy prey, but they're scarier and more dangerous than any of us! Watch out for them!" It's definitely a funny scenario to imagine.

But I don't think it would actually happen.
But I
do find it a fascinating idea of the Gang – being the cute, cuddly dinosaurs we know them as – having a reputation as dark and deadly monsters amongst their enemies. Possible future fanfic idea?

2. Are there any instances in the films where you think the gang's actions could be seen in a more sinister light?
There are a few noteworthy instances of the character acting more anti-heroically; namely Littlefoot in LBT 11, and Ducky in "Search for the Sky Color Stones". It stands out all the more because Littlefoot and Ducky are otherwise two of the nicest and most easygoing of the cast. But even there, I think "sinister" may be too harsh of a word. Even with Cera throughout most of LBT 1 – which is the meanest we've ever seen any of the main characters behave – she still reforms by the end of it. And the "reformed bully" being the whole point of her character arc in the first place. So no, I don't think there's any instances where the characters could be seen in a more sinister light, per se, or at least no more sinister than what's already portrayed.
Again, I like the idea of putting a darker spin on the characters. But I think it would largely be a fanon thing, as canon simply doesn't give us much to work with there.
Update: After reading the other posts in here, Sneak brings up a very good point in regards to Littlefoot wanting to kill the original Sharptooth. It's mitigated somewhat when you consider how dangerous of a predator Sharptooth was; not only to the kids, but also if he'd discovered the Great Valley. There's a strong "self-defence" argument to be made for killing Sharptooth, and thus in itself it doesn't make the kids come off sinister.
However, it seems to go a step further for Littlefoot than mere self-defence. It's not outright stated, but certainly implied that taking out Sharptooth is personal for him. Which I really can't blame him for. But it does indeed paint Littlefoot in a darker light, and shows a very uncharacteristically vengeful side to his psyche.