Hi and welcome.
If you have an already established character, from any kind of published Star Wars material, in mind (Canon or non-canon), it is approvable. If the character you have in mind, has not been made already by someone else in the Legion, you have to find 2-3 reference pictures from official material of the character. And when you make it, it will have to be as accurate as possible, in accordance to your reference pictures.
If you have a custom Jedi in mind, or what is called a Generic Jedi in the Legion, we have a set of standards with both obligation items to include in your costume, as well as voluntarily. I suggest you look them over. About short sleeves, the standards currently says either long sleeves or no-sleeved Outer Tunic and a long sleeved inner tunic, for the regular Generic Jedi. I am not that familiar with the Old Republic Jedi standards that the Legion has, but you can read them on these links:
List of current standards in the Legion: http://www.rebellegion.com/costume-standards/
SWTOR styled Jedi: http://www.rebellegion.com/swtor-relnex ... edi-robes/
KOTOR styled Jedi: http://www.rebellegion.com/generic-kotor-i-jedi/
Regular Generic Jedi: http://www.rebellegion.com/jedi-costume ... s-generic/
Unfortunately we don't a semi-canon category.
Concerning attending events, there are different types of events we have, and it will be listed in the signup thread what it is and what the costume requirements for it are. Unless the Event Organizer requests for specific characters, you have a fair chance at attending events.
Hope this was helpful