Scotland's tough, what with the iffy weather. Forecasts may not be accurate very far out, and a lot of tours seem to use small vans. If you wait until you're pretty confident you'll have a good day, the tour you want to take may be sold out. (Voice of experience here.)
I'd try hard not to plan to depend on tours when going to places reachable by public transportation, because the latter may be more readily available on relatively short notice. If planning to employ that technique, though, I'd recommend checking availability of the bus or train you want to take in the days leading up to your target date. I've seen buses sell out, and I assume trains sometimes do, as well. The truth is that the tourism infrastructure in Scotland seems to be lagging a bit behind the demand. It makes the country a lovely place to visit--it's generally not overrun except for Edinburgh, but it also makes getting around a challenge if you don't have your own vehicle.