I apologize in advance for the long post!
You asked me to check out your OCs, so here I am! I hope you don't mind if I give a little feedback...
As a whole, a common theme I see among your characters is power. All of your characters possess otherworldly power that surpasses that of basic unicorn magic. These characters are used for roleplay (I'm guessing), so in a MLP roleplay, these character's power is far greater than anyone around them, giving an unfair advantage that borders on power-playing. And though I'm aware that dragons are generally bullies in MLP canon, you seem to have an entire roster of antagonists, who are all very powerful. That could lead to some problems, as those types of characters tend to be overdone and people don't take to them very well. However, I'm here to tell you what I think is great and what little things you can do to make your ponies 20% cooler!
Shadow Mare and Spark
Shadow mare is a good base character. Having unpleasant characters isn't always a bad thing like some people may think! As long as they're done tastefully, they're super fun to play! Shadow and Spark's pairing was an excellent choice, especially with Spark's light color scheme and good-natured personality. Both of their designs are akin to their personalities; dark and light. Spark seems to take Shadow down a notch, as Shadow is forced to care for such a kind little dragon. Spark is also kind, though is capable of defending herself, which is good; you don't want a one-sided character. The dragonpony's magic bothers me, because it makes them alicorn-like. They can fly and conjure magic, despite lacking horns or any other magic implements. Perhaps attributing dragon-pony magic to artifacts/wearable items that can be removed would be better, which allows for some sort of weakness and doesn't allow them to be OP characters. I think that can be attributed to all of your characters. Or, remove the wings on some characters and replace that with only magic; not all dragons can fly, Spike and your baby dragons being good examples! Also, the criminal-on-the-run thing is kind of vague... Perhaps the crime was not intentional? Bad guy are kind of overdone, and I think theres a lot of potential for Shadow! Good characters have layers of flaws, and if you incorporate that it can easily take your character from an 8 to a 10!
Dawn Mare and Torch
Having Dawn be a rivalling sister to Shadow was a good choice, it allows for a sort of ying-and-yang complex; the good and bad. Even their dragons are opposites! Again, opposite color schemes were an excellent choice and reflect your characters personalities. Her personality could use a little work though, just a little! She's "pretty likeable" but all of her characteristics seem to say otherwise.. And she doesn't have any flaws that debilitate her or really affect her in any way (she's bordering on Suedom), just some things to think about!
Knight Mare & Zephyr
Both very good characters! I like Knight Mare, her personality is well-rounded; she has reason for her traits that coincide with her profession. She doesn't let her blood ties interfere with her work. Having a wise companion was an excellent choice, as a leader she has to be informed. However, 'having someone thrown behind bars because they upset her' is a bit of a stretch; kind of makes her seem like a corrupt leader, when I don't think you intended for her to be that way (though I may be wrong). I'd love to see some backstory on her, could be very interesting!
Stryker Mare and Chill
Another very good pairing; I love, love, LOVE Chill's design. Placing a frightful character with a very capable character works very well. Chill has many flaws, though I'd like to see her with some perks as well! I think she really deserves some pick-me-ups, she's an awesome little dragon character! Stryker is also pretty well-rounded, and I like her lack of magic. The fact that she makes up for this in her own time by physically improving herself adds layers to her that I don't see as much in the others. However, I think you may have made up for her flaws a little too much. She's skilled with any weapon, meaning that she has practiced vigorously with every weapon ever designed. It takes warriors years and years to perfect their skills with merely one weapon... Potentially a lifetime! I would perhaps refine this to one or two specific weapons that greatly compliment her character, making it more believable! :>
Again, this isn't me saying 'CHANGE THIS!', it's merely my observations: what I like and what could be done to make your characters even better! :> I think they're all good, there's just some little bits that can be elaborated upon and improved! :>