Well, my first Major (30+ years ago) allowed me to sit my then fiancee on it and wind it up with her sitting on it. I wasn't too concerned about vibration, obviously. My current Major will just about hold my 12x15 inch Universal tailboard, though it's at the limit. It's not a weight limit, just vibration. I'd say, realistically, if you're worried about vibration, 15-20 lb., but a lot depends on the camera design.
Minors, dunno. Never owned one.
Cheers,
Roger