High passes don't become snow free until late June, early July. I remember once going to Black Bear Pass in August and there was still a big snowfield in the way at the top -- had to go back down the way we came up. I remember Ophir pass once with 12' snowbanks at the summit (someone had dug through) in July. But usually the snow is off the trails by the end of June.
Anyway, southwest Colorado was always my favorite place to drive the trails and passes - Engineer, Stony (it's where the Rio Grande River starts), Cinnamon, Black Bear (I never did get over it), Ophir and Imogene are ones that quickly come to mind. And lots of interconnecting trails and ghost towns. New Carson on the east side of Cinnamon pass was in really good shape when I was there in '96 (interestingly, in a Cherokee - XJ). Pick up a San Juan National Forest map for the area.
An interesting side excursion is north of that area - Owl Creek Pass (you can take a car over it). On the west side on the way up is the meadow where the original True Grit movie filmed the final shoot out.
I don't know about tours - I always went with family.
Other maps to pick up for western Colorado if you're not doing the San Juans - Rio Grande, Gunnison/Uncompahgre (Owl Creek Pass), White River, and San Isabel (if you're going to be near the Sand Dunes NP). They've consolidated some of the maps since I last bought any (80's and 90's), such as Grand Mesa, Gunnison and Uncompaghre.