Thanks. It feels great to be doing this again. Hopefully I can make more of these in the near future.
1. I see what you're saying. I believe AJ meant age; how many times have you heard the phase "when I was little"? While it can be taken for size, often not people use it to refer to a stage in one's life. So when AJ says "even littler" than the CMCs, she refers to her growth state in life and being younger. And if you think about it there's no point in AJ comparing sizes within the same ages when someone with a large enough growth spurt like Big Mac is around.
2. Applejack has never been shown spending money. It's not from a lack of wanting either; from the Gala events we know there are things she wants but never had the bits for. Even when promising it she had to get it somewhere else instead of pulling from her own pocket. No matter what the farm takes in it seems AJ is handed down money rather than taking from the top of the pile. Recall the only person we've seen in the family put forth large amounts of bits is Granny Smith.
3. As for responsibility, AJ's main role looks to be that of stall seller, with her helping around the farm in other tasks as a secondary role. In Applebuck season Big Mac headed the role for applebuck season with AJ learning about underestimating the job when he handed it down to her. Granny Smith does all the zap apple jam making--one of the two big income pulls--all by herself. And cider making is a team effort. AJ still organizes, though she's more new to it and has been overruled by Granny a couple of times.
So while I agree AJ more than Big Mac is moving to towards taking over the farm, when mlp started she was more third string there and was still quite inexperience at parts of her own job. Compare this to Rarity, Rainbow and Fluttershy who had already settled well into their roles and in two cases have moved up the chain.
4. I think there are enough clues for a guess. Sweetie Belle was five during her birthday--and half the size--years later SB is still a filly, just learning magic and going to school. How many other species can you think of that has about tens years go by and not have the creature in question grown up by that point?
5. Maybe, but Trixie doesn't know AJ personally to know what insults would stick. And did she refer to Twilight, Rainbow and Rarity the same?
Despite my arguements I can't say for certain where AJ would fall exactly in the group--Twilight is hardest to guess with privilege and studying masking the clues that would help. Though I don't think AJ would be in the upper end where Fluttershy and Rarity would be. Pinkie Pie and her would certainly be near each other age wise if I'm right.
Visual media like this is about showing, and since heavy mystery isn't the point of mlp I think we were meant to see the 21 candles. Otherwise why 21 and why not just do the big one candle thing other shows do to get past listing the age? 21 matches Rainbow not being in school and gives enough time for her to make the upper positions of the weather team.