The American Girl figures are closer in scale to Monster High (monster high has longer legs though). The arms don't have elbows, but arms and legs do work with other figures, the limbs are a bit longer than Halo/TMNT, maybe more similar in length to CoD (I cannot say for sure as I don't have any to compare with currently, I've bought many of sets for my nieces).
The heads of American Girl and Monster High are exactly the same. Torsos are much larger than Destiny females and TMNT April O'neil. For example, if you stand April next to an American Girl figure, April looks like she could be the other figure's doll.
What does work perfectly for swapping is the feet/shoes of the American Girl figures to the Ninja Turtles feet/shoes. Although so far the only turtle with shoes is the "Mutagen Canister" Mikey.
Hope this helps.