The KV slides are side mounted. They can be attached on the inside edges of the cabinet, or you can use the face frame mounting brackets along with some rear mounting brackets at the back of the cabinet. This eliminates the need to flush out the cabinet sides using wooden blocks or build out strips.

For the depth, sounds like you have an actual interior cabinet depth of 20.75″. I would add a 3/4″ cleat on the back of the cabinet to ensure there is enough support to hold the drawer weight. That gives you 20″ of depth to work with. 18″ would be the best option for the depth of the slides and the drawer box in this situation.

https://www.barkerdoor.com/KV-drawer-guides-and-slides-s/113.htm

https://www.barkerdoor.com/Face-Frame-Mounting-Bracket-for-KV-8400-pair-p/kv-8404-(pair-of-2).htm

https://www.barkerdoor.com/Rear-Mounting-Bracket-for-KV-8400-pair-p/kv%208403%20(pair%20of%202).htm