There is like the simplest solution here:
*Just don't comment if you don't think that person has participated enough in the forum*
You can get all ratio-y and 'you must post x before you can start a topic' but that'll just drive people away- I know this because I came across a lot of forums before this one where those rules did apply and I just left before I'd even finished signing up.
I never read the rules before I first posted, I didn't even know we had rules until like last month, and I turned out fine
I think it's just comment sense, no one's forcing you to comment on anyone's work, so if you don't want to comment then just don't!? If people really want to join our amazing family and become forum regulars they will realise it takes forum dedication