I don't know when the Black Watch tartan was first woven, so I can't help you here, sorry.

As far as the kilt vs. great kilt thing, I have a very reliable suspicion that unless there are some Xmarks at the event no one will ever know the difference. I would go with what you got, unless of course you like the great kilt over the wee kilt.

As far as the reunion outfit. I would make sure that the tie is one of those pencil ties, since regular ties will look rather fat when worn so short. To go ever further, I would say that unless this is an extremely special reunion you might be dressing up a bit too much. Heck, your proposed outfit is good enough a for wedding, which isn't bad; there just isn't any reason to put on more than is appropriate unless you like the look. Heck, the kilt, hose, button-down shirt, black shoes, black sporran, black belt and tie are plenty for such an event. If your sgian dubh is unsharpened, then I would leave it at home, because it is then an ornament for weddings, funerals, and piping. If, however, your sgian dubh is nice and sharp, then I would totally wear it, because the real ones seem to look better than the unsharpened ones. At the very least, the fly plaid might be over the top, almost on the costume and superficiality side.

Keep it simple and sharp and no one will be distracted by anything other than your kilt and its pleats.