You know taking a second look I've noticed some, not problems, but things that bother me personally with some of the designs.
Gilda: I don't think Gilda is designed badly, but I just don't think it fits with her depiction in the show, maube she should be a little taller, a little leaner, smaller breasts, and slightly darker skin tone, and sharper, more bird like features for her face, she's a griffon after all, maybe give her a lean upper body and stocky legs.
Scootaloo: Honestly her face looks like it belongs on a grown man, everything elsle, well maybe give her shorts instead of long pants, and make her t-shirt orange and blue.
Big Mac and AJ: The only thing that bothers me is that they're to pale, they're farmers after, they're out in the feilds in the sun all day.
Flim and Flam: I find it a little odd that the height difference btween them is so prominent since they're brother,s and implied to be twins, in the show, but it actually doesn't bother me, it's just kind of weird to me.
Twilight and Trixie: Neighter are designed badly, but I think their designs couls use a bit more contrast because Trixie was meant to serve as a dark mirror for Twilight, what she could become if she let her ego get the better of her. Maybe have Trixe dress a little more sududtively, or give a more eatheral look to her to better contrast Twilight's down to earthvclothing.
Well there you have, those are the thing that bother about some the designs, don't get me wrong I still love all of your designs for them, but I think some could be better.
On the matter of Big Mac and AJ; considering that both of them freckle, it's unlikely that they are even capable of tanning whether or not they spend a lot of time in the sun. If anything, they would just have more freckles. One other thing is that apple usually grow best in colder climates where there would be less sunlight than more tropical areas. However, I do acknoledge most of your analysis, and agree that these designs are very beautiful.