Answer: The normal height for a Canadian hemlock is 40-70' so trying to keep it 7-8' in height will be very difficult! These trees do take well to shearing and are often used as a dense screening hedges, but it takes regular pruning to keep them as small as you want them to remain. When they're sheared, they're usually kept at about 20'. I think you'd have better success with Tsuga canadensis 'Pendula' (Sargent weeping hemlock) in the site you describe. This hemlock grows to 5' tall and about 15' wide.
Best wishes with your landscape!
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