I'm not sure Aggie , it sounds lovely though like a beautiful area!! I would hope your landscaper knows enough to recommend a good plant for you. Give it a try! :)

Other than being marginally hardy in zone 7, the growing conditions sound appropriate for this clem (assume it is C. armandii 'Snowdrift'). It is quite happy in quite a lot of shade and would likely require it in such a hot climate!!

How large is the trellis? Clematis armandii is a BIG vine......one of the largest growing clems IME, and 4 may be overkill unless a very large trellis. The one in my inherited garden grows up into the top canopy of a 40' horse chestnut tree and then hangs down a good bit. It is easily 50' long!!