It's my understanding that guides are not chosen for particular dates until after January, and are subject to change. We have had one guide who only worked one particular tour, but several times a year. Others work multiple tours. They have a lot of leeway on how many tours they can do per season.
For what it's worth, we have taken 12 tours, had 10 different guides, and only one of them was not excellent.
I'd have to go back and check my notes to see about bus drivers; all the bus drivers we have had have been excellent as well, but they vary in terms of how much they interact with the tour group. Do be aware that the driver is considered to be part of the group. Some of them join in on excursions and meals; others not so much. A great driver adds a lot to the tour.
We did have one driver that was not well liked by many of our tour mates. DH and I liked him just fine, but some people got upset when he joined us at dinner and had a beer. That may have been the snootiest group we ever had the pleasure of traveling with.
Note: EU rules about drinking and driving are very strict. The driver I'm talking about was off duty; there was no bus time that day.