Keith Newton
It isn’t mandatory that the sides be 5/8”, they could be 1/2”, or maybe 11/16”, but above that might rub somewhere.

As for the expansion, there are LOTS of holes in the sides that screw into the cabinet, but no slots. However if you got a thin jigsaw blade, you could saw between two to make a slot.

The length of the drawer is critical because there is a prong at the back of the slide that holds the back, and allows adjustment for twist.

How deep is this cabinet going to be?