Keep in mind both of these are not the large-flowered clematis you see in nurseries. A picture of on is linked below. Go to "clematis search" at the top to find a picture of the other. Full sun or part shade for both. Addisoni gets 1-1.5m tall, pitcheri 2-2.5m. Plant them now, keep them well-watered, prune them back to the ground or a set of buds around the time forsythia bloom.

Here is a link that might be useful: c. pitcheri

I'm not sure how you got the impression that clematis are "extremely" difficult to grow - they are no harder or trickier than another type of plant, as long as you are careful in their planting and have a bit of patience. It is true that many of the large flowering hybrids can be problematic with establishment (including the chance of the dreaded "wilt"), but that too is not insurmountable. Starting with the species forms, such as you have already selected, is a great idea, as they are often pretty much care-free, but don't be discouraged from trying out some of the other types with showier flowers. They are quite easily grown as well, as the many posters to this forum will attest and can be quite addicting!