That she, as a pegasus, can't fly could be considered a disability. I think it'd be similar to being stuck in a wheelchair only not so bad in this case as there are many MANY other ponies who can't fly and are born that way so she never has to feel left out except where flying is necessary to an activity. I don't know about her family but if she comes from a long line of pegasies this could potentially be very traumatic because her family can do things she can't and she might miss out on a lot of family bonding. Of course, most, if not all, of this is null and void if she's just an anomaly like the baby Cakes in which case her parents could be anything. This could be another explanation as to why she can't fly normally. Her pegasus genes might not be strong enough. That's just pure conjecture mind you.
It's a lot to think about, but the truth is, so far as I know, there is no official explanation. Scootaloo is Scootaloo. No backstory.
I will try to explain (my english is so awful). I think any photoshop can do the job pretty well, and try any kind of effects and filters, you know, highlights, shading, textures, anything, and you can create awesome effects with experimentation. The ponies are easy to draw, but check the proportions of the body, and remember it. Practice a lot, the perspectives and shapes, it is very important. And of course, try harder every time, keep up and you can get better and better by the time.
I am really loving your style as well as how the character design really represents scootaloo's personality! The shading and brightness of color are also really well done! Would love to see more! Great work! Kind regards, Princess Luna