Ruck training

Given that we have a choice as to how many players go into a ruck, defence and attack, does the game always designate rucking to certain players? If so, which numbers?

Likewise does the game assume that all forwards participate in scrums or is it only worth training the front row for example?
With regards to rucking all players may be involved albeit the backs less so.

As for scrummaging unless you put your winger in as a prop the scrummaging points don't matter. If you do put him in the front row he might snap though. All forwards scrum points count. I've no idea if the front row matters more than others however.