Being the writer of 5 CRL's, may I say, go for it!! Make the costume as best described. First, find out if the character belongs in the 501st(a character in a dark seedy bar with no background story doesn't necessarily say he's a bad guy.
If there is minimal reference, use your best judgment or ask the members, we're here to help!!
Once the costume is made, breakdown the costume like similar CRL's. Write out the CRL in three tiers. One for entry level and two for more detailed attributes and experienced costumers. I can be as simple as "pieces are made out of aluminum." Or more precise like, "a 7/8" gap between stitching is a must..."
Once pictures are made of each piece, and front back and side pics are taken of the costume, submit it to the DL. The Detachment Leader will send it to the Legion staff. And there will be some changes and rewording(what sounds good to you, might be confusing to others). All in all, the Legion loves new costumes. It takes members like you for the growth to happen!