How often you hit shoulders all depends on many factors: program, diet, genetics, fitness level etc. I you are trying to just tone up and limit your workout to high rep/light weight and rarely reach failure, then you would probably want to hit shoulders more than once a week. If you blast a muscle group going to failure during core and other lifts, then once a week is probably enough. There have only been two points in life when I could handle hitting my muscles hard twice a week. One was in college when I had two hours a day to spend in the gym for a few years and got in excellent shape naturally. I've also been able to do it again lately on a TRT dose of test. But my current workout isn't has hard because of time constraints. I don't have time to spend over an hour on one group in a day.
As for shoulders and pecs, those are two I would never combine. My anterior delt can't take that abuse. I have to put them as many days apart as I can. I usually combine pecs and tri's and shoulders have their own day.