A no asshole rule is bullshit. What you want is a no bully rule.

Being harsh is alright. The people are respect the most are those that have the balls to come up to me and say "your idea is rubbish, your point of view is stupid". Yes, this guy is an asshole. But he is a good one.

What you want to avoid is the bullies. The ones that will insult you for no reason. Belittle people to feel better. Those are the ones that are dangerous for a team.

Everybody is an asshole from the point of view of someone else.