We use "Ya sas" , when we talk to many people or when we want to be polite with a person (no matter if the person is a woman or a man). In other words it is the plural of "ya sou" and it's the polite form. We use "Ya sou", when we talk to a child or a person that we already know good. Usually we say ya sou when the other person is younger or has the same age with us...

When you meet someone for the first time,yassas is the best welcome. If you have seen this person twice or more and he has become a friend of yours,gia sou or yassou is the best way ;)

for what i know, first form is second person informal (thou) and last 2nd person polite. (i think the femenine is missing but not sure).