The hardest thing to do is differentiate between handwriting when one person is writing. I see a lot of similarities in these signatures, particularly the 'looping'. To be honest this style you used looks good for a Rarity or Fluttershy signature. Fancy but still down to earth, when it comes to the others there needs to be less looping.
All in all not a bad try, I have done some experimenting with signatures and noticed I loop a lot too. It's why I was able to see it here.
As a thought, here is some analysis of the ponies themselves that might help in differentiating them in future work.
Pinkie will be bubbly and full of hearts, I think a bit much like a teenage girls handwriting.
Rainbow Dash wants to be famous. She would have practiced her signature for when she writes autographs and such so she would have two distinct looks to her writing. One is her normal signature for her personal use, and the other a 'quick' style for writing her name fast on a piece of paper hoofed over to her.
Applejack is basic as basic gets. She would not use cursive I think, opting for a normal hoof written signature.
Twilight is a perfectionist, and would go for something that Princess Celestia would have used. Think old school style like John Hancock and the like in the Declaration of Independence. Maybe something from some old English works.
Fluttershy is hard, simply cause she is a mixed bag when it comes to personality. I think a combo of Loopy and cutesy would go well together. Maybe a simple cursive cause she would be too shy to have a complex one?
Rarity I think you go spot on. Fancy but not old school with a touch of flair.
My thoughts on all this and I think you are on the right track with it, practice makes perfect and typography and calligraphy are some of the most difficult artistry's out there. Nice job.