Breathing squats are systemic in nature, they efect your entire body inside and out, as well as your mind. For that reason you simply cannot over do them as so many try to.
It is easy to overtrain on a breathing squat routine by 1)doing too many sets of the squat, 2) doing too many other exercises, 3) trying to do other heavy exercises along with the squats, and 4) training too often.
To succeed you have to specialize on doing 20 rep breathing squats, everything else is secondary. You must complete the twenty reps no matter how hard they seem by exercising your will power, and still add five more pounds the following workout.
Because they are so demanding one set is all that is required. Two sets are just too physically and mentally demanding. You need to recover between workouts so two days training per week is nescessary. Eat a full weight gaining diet with lots of protien. Do a few light exercises to stimulate upper body growth, nothing too strenuous. Expanders are good in this case if you do six to ten exercises one set only using 12-15 reps each.
Every strength athlete and body builder from the 30s, 40s and 50s (the pre-roids era), used breathing squats with success. Doug Hepburn, Paul Anderson, Steve Reeves and Reg Park all used them with great success in their chosen field. I went from using 125 LBS to 250 LBS for 20 reps in the breathing squat and gained 6 inches on my chest.
Give them a try. They still work today, that is if you work hard enough at them.