Since your garden isn't, you say, large, I really recommend at least half the plants should be evergreen; otherwise you will be looking at a collection of dead twigs for 6 months of the year. (and that includes I'm afraid Clematis Montana) There are evergreen clematis, but I believe that they would not tolerate v. cold conditions. What part of the country are you? I am in East Anglia and have had an evergreen clematis that did almost too well. Mermaid is a fairly evergreen rose, I expect there are others, I always go to rosebuddies.com for advice. Good luck with it and do keep us posted.