The main reason I recommended splitting your quads and hams into different days was because in your other routine, you were working all leg muscles several times a week. Since your goal is to increase mass, I figured splitting quads and hams would be one way to give you multiple "leg days" but also allow more recovery time.
Squats are an all-encompassing exercise, hitting quads, hams, glutes, adductors, abductors, and even calves to some degree. That being said, I'd still probably do them on quad day. I don't think there are any hard and fast rules though.
Another routine you could follow would be two full leg days a week, one day heavy, one day light. Right now I do legs one day a week, alterating heavy one week, light the next. There are many variations. What works for one person may not for another. I switch my routines/workout splits every 6 weeks or so.
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