Let me offer my thoughts.
Γεια σου means hello in singular. I mean that if you meet someone you can say γεια σου!. Γεια σας is used for hello again but in plural. When you meet your friends you can say γεια σας. Χαίρετε means γεια σου again but it is a little more formal but you can say it too daily.
I hope that was helpful.