Overall, I think you have a really good start to your ponies. The only thing I would recommend is cleaning up your lines and edges a bit because your lines tend to be a bit wobbly and messy. In the cutie marks and designs, it doesn't look too bad because it gives it kind of an artsy look. But around the eyes, it looks really messy.
You should practice making delicate lines and get better brushes if you don't have good ones. It doesn't matter how tiny your brush is, if it doesn't come to a razor sharp point, you won't be able to get fine details. So good, synthetic brushes in small sizes, that come to a really fine point are a definite must.
Since you don't seem to be doing full body repaints, you can also clean up any globby, wobbly, or messy lines--or make the eyelashes come to a finer, more delicate point--by taking the edge, or corner of very fine sand paper, and using it to gently reshape the lines. I've used this method many times and it works quite well for cleaning up.
Otherwise, your customs are quite cute. You've started off far better than many beginners, which is something you should definitely be proud of. =)