the main difference travelling on a Sunday is that there are a lot less hours to choose from...but it is still very do-able. Just keep an eye on the schedule, and be sure the last bus runs on Sunday, otherwise you will be sleep in Piazza del Campo :-D
Yes, the "festivo" is for Sundays and for national holidays, so for a Monday or any other day of the week that is a holiday, that same "festivo" schedule would apply.