In my personal opinion, I think a ruckman is unique and a past ruckman would be best as coach. Generally a forward coach, defensive coach etc is all about a structure. Whereas a ruckman is more about a specific group of skills for a specific position.
I suppose it’s similar to clubs employing an accurate goal kicker to help forwards who struggle with set shots (ie. Lloyd, Dickson etc).
‘I understand it is an opinion, but it is not a fact.
Nobody apart from those up close and personal with Rocca know whether he is a good ruck coach.