what is a good rule of thumb for what order to prepare dishes - longest cooking time first, right?

Yes, do the dish with the longest cooking time first -- but you may also have a couple of dishes, like the root vegetable gratin, that can be made in advance. For the salad, I'd wash the greens and make the dressing a day or two before. Then you only have to do the turkey, stuffing and brussels the day of (and I'd do them in this order). Have a great holiday!