I would like to plant a quick growing privacy hedge and found the Hybrid Salix. I am concerned about the eventual width (we really want about 12+ ft of privacy, not too much width). Can anyone offer advice on this tree or recommend another easy-to-grow tree for an inexperieced gardner?

For privacy hedges you have more than a few choices. American Arborvitae, which is pretty tight when planted to make a hedge. 5' wide base but is slow growing
Canadian hemlock, somewhat fast growing and can withstand large amounts of trimming and sculpting to your width preference.
Same with the hardy amur privet hedge. I do not know the width of the forsythia bush but if you prefer a hedge that flowers that would be a decent growing one. it can be trimmed after it flowers, same with spirea (which does not grow as high but grows a good 5-6')
If you do not like the Idea of having to prune then you can go with the Green Giant Thuja. The benefit of this is it grows 60' tall if you want. makes a great windscreen and it is evergreen so you have it as privacy all year long.