Also if you headbutt someone during a tackle and knock him out , do you get penalised or is that "play on"?

shirt pulling is called jersey slinging and is illegal in rugby union you will be penalized for dangerous throw and will problably be yellow carded and if you headbutt someone in a tackle you will most likely be penalized for high takle and be yellow or red carded dangerous actions are not tolerated in rugby union

penalty,yellow or red card

Shirt pulling is ok but they are quite tight, so it would be more effective to actually grab the person instead. Headbutting is penalisable as far as I know. Besides why would you want to headbutt someone? You know you could harm yourself.