In 15s, in the case of kick-off fouls the opponent team can choose centre-scrum or kick-off repeat.
In 7s, in the case of kick-off fouls centre FK is given to the opponent team. Not sure about 10s.
I believe that the rule should be unified by 7s' FK.
7s is more a possession game where as 15s is more territory based. 7s you only have limited time so keeping possession going backwards to find a space is better than losing the ball.
15s going backwards in your own half is pointless, its better to get it down field and force your opponants to make mistakes.
Is it possible to merge RU 7s and RL 7s? Since RL players often play RU 7s (like Papua New Guinea), there is unlikely a big gulf between RU 7s and RL 7s.