If the one-gallon can is full of roots, the mass is large enough to go into the ground. When I buy them locally, I buy the ones with roots creeping out of the drainage holes in the bottom.
Yes, if you want to keep it in a pot for a long time, you should pot it up gradually, the way you do with your roses, but it all depends on your reasons for keeping the plant in a pot. If you think the plant will get lost in the landscape and not receive the required attention, you could plant it next to a rose or another plant that does get regular attention.
I'm growing about 25 clematises in partial shade and more than 100 in full sun. I can't say that the former look better than the latter. Full sun is fine, as long as you keep the bottom shaded, but there are various ways of doing that. I had a nice collection of small pieces of shale left over from a fish pond project, so I cover the root area of the vine with shale.
