Should I add salt to my sauerkraut recipe if I add vegetables?
If you are fermenting sauerkraut, then you should keep the salt content the same if you add other vegetables. The salt helps keeping undesirable bacteria in check, so you probably don't want to "dilute" it. Increase the amount of salt so the ratio between cabbage/vegetables and salt stays the same.

TL;DR: Treat any additional vegetables like cabbage when calculating the amount of salt.