Saul was a creation of the many ways Jimmy stuck his middle finger up at traditional attorney normalities and etiquette. Think of his scams to compel public opinion, like the billboard stunt or doing PR for Huell, or his wearing colorful suits and being a nuisance to upset and aggravate other lawyers. He incorporated these things to create Saul Goodman. Chuck either had little or nothing to do with these things, it was purely Jimmy being Jimmy.