It's supposed to stick the saddle to the horse basically . I guess if you're using it primarily for cushioning you could use a saddlecloth underneath. I don't find the limpet needs washing often but when it does you don't need much detergent and never put it on the spin cycle in the washing machine because they tear if you do. This means they take ages to dry but they're such good pads I don't mind this.
If you get the thicker one I can't imagine you would need anything thicker unless you were trying to get a very badly fitting saddle to fit .
I had one thick one it was too thick for our saddles .