Hugh, if you have a fair amount of space behind your listening position for a back surround effect to develop(likely, since your room is 19' long), then two back speakers can be worthwhile, especially if your receiver has DPLIIx to steer ambient sounds contained in two or five channel material to the back speakers. Some prefer to use more direct radiating speakers, such as M2s, for back surrounds, rather than additional QSs.
16ga speaker wire is fine in most cases, except for very long runs. If you haven't already done so,
this article on speaker wire by a well-respected veteran audio professional should be studied.