I've entered RSOM a few times, and lost every single one....but besides that, here's some tips.
A long story will NOT fit into 15 pages. It just does not work at all. You need time to establish the characters and situations, the begining, middle, and it MUST have and end, otherwise it isn't a short story, but just a chapter that gets cut short. Better to tell one particular incident in a character's life than explain a large chapter of a life in so few pages. A story can start off with an arguement, but if it's carried through the majority of pages, you need to focus on resolving that before the story ends. A short story can't afford to branch off like long stories do, keep it short and sweet.
Art wise, you need to work on anatomy and backgrounds. I thought it would be somewhat passible looking at page one then I saw page 2 and noticed how bland the backgrounds look.
STUDY FROM LIFE AND OTHER ARTISTS.
That is the best advice I can give. Cliche, but true.
Back when I was like that I put off entering for another 2 contests to improve my skills. I noticed a considerable difference in my first entry compared to my last one. Even if I didn't win, I now I improved and that was some comfort at least.
Also, I see nothing wrong with getting inspiration from other styles, but they really do look like certian established charaters, TOO much so in fact, you may run the risk of disqualifacation, IMO.
Good luck though. Even if you don't win, you can get good practice from this at least. I have no interest in continuing to enter these contests, but I learned alot about pacing, writing, and managing my time through this contest.