Securing French cleats to drywall over lath and plaster with structural screws
My guess is that the screws you want to use won't work well if the shelves are heavy.  The embedment in the stud is too shallow and the screw will flex quite a bit.  I think the shaft of the screw is too small for the drywall and plaster to provide much support in shear.  

I'd simply strip off the drywall and screw 3/4" plywood on top of the plaster.  I'd nail on a pressure treated 2x4 like a base molding so the plywood isn't in contact with the floor, and rest the 4' edge of the plywood on the 2x4 before attaching.  This way the weight is carried by the plywood rather than the nails.  

With that in place mounting your cleats will be very easy.  The plywood will also hold up better than drywall behind the shelves.  It's a bit of extra expense and effort but you'll have a very strong and durable and nice looking finished product.