My kitchen staff never touch the POS. Do I still have to add them?

Only if you want their hours. The kitchen screen never asks anybody to sign in, so a cook who only reads tickets needs no PIN. If you want cooks on Reports → Hours worked, add them under Settings → The POS → People at the POS with What they may do set to Sell, and they clock in on the sign-in screen of the counter POS or a waiter phone on their way in. They never need to take a sale. Nobody is charged for a person, so adding the whole team costs nothing.