The "rules" for prom in the US differ quite a bit from typical black tie (which is, in my opinion, unfortunate). Matching of your date's dress has become the accepted standard and your date, undoubtedly, expects you to do so.

So wear the teal tie, don't worry about whether it matches your kilt or not as most Scots seem not to worry much about matching, and have a great time!

Your outfit sounds great, and your choice of sporran sounds like a good one; though there's no way to tell without pictures. A leather hunting sporran with a silver cantle works fairly well for both daywear and black tie.