Tipping in Scotland is generally accepted as 10% in restaurants and for taxis. If you receive exceptional service, tip a wee bit more. Bar staff appreciate a small tip, but you're under no obligation. As with anywhere in Scotland, check your bill at a restaurant to see if a service charge/gratuity has already been charged. Restaurant front of house staff in Scotland generally pool their tips, and share them with the kitchen staff/bus persons
As a former tour bus driver, I can honestly say that any tip is appreciated, but if a tip is not already built in to the total charge (and it shouldn't be), then something along the order of five to ten pounds per day per person would not be over the top. That would be the case in a 6 to 12 passenger minivan, especially if you're getting narration in addition to driving. In the case of a huge tour bus, a whipround at the end would be more than adequate. That said, tipping your driver/narrator is not obligatory.
Hope that clears things up a bit.