I would advise against wearing a tartan tie with a kilt-- as Jock Scot and a host of others have pointed out, it's tartan overkill, especially when worn with a tartan kilt and tartan waistcoat. I'd recommend joining the Royal British Legion (as I understand it, membership is open to anyone) and wear their tie, or contact Jock Scot and see about obtaining an Army Benevolent Society tie-- again a "British" tie that can be worn without causing adverse comment. Other than that, stick with stripes and solids (your college tie is always a safe choice) and you'll look fine.

The hunting sporran is more than acceptable for dressy evening wear; I usually wear the same sporran day and evening, and only trot out the silver mounted seal skin if it's a formal (ie: black tie) event.