iflyhigh605 - Welcome to the 405th! Technically, you are unable to represent the 405th in a costumed capacity until you are the age of 18. Until such time, and once you have more than 50 posts and 90 days here, you can represent the 405th at conventions/events under the supervision of a fully deployed member as a handler etc.
Having said that, DragonCon is in fact a little different and we would somewhat default to the Conventions rules for the parade. The parade will always stick like groups together. Therefore, anyone who signed up for the parade and indicated that they were going to march in a Halo costume would be told to go find the other Halo guys to line up whether they are 405th or not.
This next bit of information is not so much for you, but for anyone else curious now that I've said that about what we would do in that situation. I can tell you it's been happening for years for the 501st. Typically it's not just a person or 2, but several, so we put them out in front of the Rebel Legion and then we're behind the Rebel Legion. Basically, mixed Star Wars people, RL and then the 501st. SO.....if it turned out that we got a lot of non-405th members showing up for the parade, we would simply put our banner and members behind them. No problem at all. And we'll take the opportunity to encourage those people to check out the 405th and see about joining our ranks!
Now, back to you, iflyhigh605, let's revisit this closer to DCon. I'm assuming that you'll be attending with your parents or some other adult so we'll figure things out based on several factors. In the meantime, keep participating here on the forums, get to know the Regiment in your area once you qualify to be added to one (50 posts and 90 days) and enjoy your time here!