Many more knowledgeable than I will jump in but I'll offer this to start.

I would suggest a black Argyle jacket with a white dress shirt (off the rack) and a bow tie. You should post up right away what tartan you are wearing so others can suggest colors to match to it. The bow tie, for instance, could be color coordinated or black as could a pocket square. Just Google images of the Argyle jacket and you will get many great ideas of the look.

Nothing says formal better than the kilt with a formal jacket and I know you can rent jackets from a number of kilt purveyors but you need to get going on this if the prom is this spring. Get your hose from one of the casual kilt makers - they tend to carry some less expensive, but perfectly acceptable options - again go with a color or with black. Flashes are a nice touch and not too expensive.

Your choice of the kilt is a GREAT one and I congratulate you on your obvious good taste. And the girl is sure to love it!