Hi, Lyn. I never add compost or manure to my soil when I'm planting clematises, and I also add amendments only on top, so even though my soil is radically different from yours, it is possible to plant the clematises in this way, especially if it has worked with your roses. Drainage is the key, and if you have good drainage, you've won half the battle.
Mulch will, indeed, keep the roots cool. An article to be published later this week will illustrate several ways of keeping the roots cool, including groundcover plants and even rocks.
I'm in zone 9 and I haven't had trouble growing any clematis, so I'd say they're all heat tolerant. I don't have many days with triple-digit temperatures, but they do occur and the clematises have not wilted from the heat yet.
Under your conditions, you're right to keep plants out of the ground until they've developed a large root mass. Some mail-order nurseries do sell clematises with a large root mass. Silver Star Vinery is one of them, and it's having a sale right now.
http://www.silverstarvinery.co...
