As far as I'm concerned you look fine, although I'd suggest you ask your tailor to lengthen the sleeves by extending the cuff using either the excess material from the tails or by adding a velvet band at the bottom of the sleeve. Either will work well and either option is "proper". I also think the plain sporran works well; discrete and understated. Since it's white tie I'd go with a white waistcoat, the same as you would wear with ordinary tails.

Despite what others may say (and they are entitled to their opinions, misguided though they may be) avoid wearing white hose. Tartan, red and white diced, or a sold dark colour hose are traditionally worn with formal Highland attire.

Enjoy the ball!