They are all also socionics Delta quadra personalities. Using the MBTI labels though, this comes out as

Limestone — ESTJ

Maud — ISTJ

Pinkie — ENFP

Marble — INFP that only looks like an ISFP (Fluttershy) but isn't 100% match. Both are Fi dominants but have a different orientation of the sensing / intuition axis. My basis for INFP is that Marble, being extremely shy and introverted did not show off her axilliary Ne or Se function, but did show off her tertiary function off Si (tradition/recalling), not Ni (future envisioning/new planning). Thus INFP.



For comparision Fluttershy is also an extreme introvert, dominant Fi. But coupled with some Ni so she hopes for a new future, and a fair amount of Se in her leanings for aesthetics.