I recently was the seneschal for a scenario in which a full grown hef tackled someone, who fell back onto another character. The one that was tackled was wearing full plate. how should I have determined the damage done to the guy on the bottom. On top of his leg was a full grown hef, and a large man in full plate. How would the damage for that be determined?

Use the falling damage mechanic. Falling *on* armored people and having armored people on *you* is the same, from an einsteinian POV. Apply crushing (blunt) damage every round thereafter.

But what about weight? The Hef and the person didn't fall far but they were both very heavy. And underneath the smashed guy was snow so how would that work?

Use the mechanic, but you'll have to adjust the total damage rating as you see fit. Man/Hef are heavy? Raise the damage. There's soft ground? Lower it. The falling damage table isn't going to be of much help other than reference.