stud wall

Hi, so the french cleats on my kitchen units are not going to match up with the studs on my kitchen wall. I am going to cut out the plasterboard and replace with ply and screw the units on to this. However it is just hardboard backing on the units, will screws through this in to the ply hold?

I would like to put a couple of shelves on a stud wall to support approximately 12kg of weight (an electronic keyboard plus a few musical effects units). I wanted to use an upright & bracket style of shelf, with a small pair of uprights and the 2 shelves approximately 30cm apart vertically. The...