Nopony actually put Limestone Pie in charge of the rock farm. She just kept getting bossier and bossier until she’d basically taken over.
Rarity and Sweetie Belle have had a Hearth’s Warming tradition for a few years of Rarity giving Sweetie Belle an unfinished gift so they can finish it together.
Pinkie’s parents didn’t start talking like that until after Princess Luna returned.
While she specializes in rocks, Maud is knowledgeable on other geological topics, such as fault lines or volcanoes, that may become relevant during the study of rocks. She even has a passable knowledge of fossils, though more centered on excavation and preservation than on how to identify what they came from.
Everypony who sells candy in Ponyville knows about the colt with the big teeth, and they’re all unnerved by his habit of crunching through jawbreakers without sucking on them first.
Gummy isn’t a baby, he’s just from a species of really small alligators. It’s better this way, because he won’t get too big for Pinkie to keep. His lack of teeth is due to a genetic defect; normal gators never stop growing teeth, which is one reason Fluttershy and every other expert who’s looked at him think her probably never had any to start with, in which case it’s lucky he found somepony to take care of him sooner rather than later.
Gummy’s personality reminds Pinkie of Maud a little bit, which helped her to get through some homesickness early on.