lets say you barre a chord, your index finger will be fretting more than one string right? Ok, your other fingers needs to be as perpendicular as possible so that they do not mute or touch adjacent strings.
It doesn't really matter where your thumb is as long as the chord sounds clean and that you don't feel any pain while doing it. Place your thumb where it feels right and sounds clean. If you have small hands and try to get your thumb on top you'll end up muting adjacent strings and you'll start hearing buzzing sounds.
I don't believe in a right way to do things as long as it sounds good and feels good it should be right. Watch your favorite guitar players and you'll see that they all play differently.
As you progress in your playing you'll see that some hand positions gives you cleaner sounds. You'll eventually adapt your hand position to what you wanna play.
Always remember that your hand needs to adapt to new ways of doing things so give yourself time for your muscles to adapt. Remember how fast you were getting tired when you first picked up guitar, how much longer can you play now before getting tired.
Music schools are really good for giving you "normal standards" but in the end you have adapt some of those standards to yourself. They don't fit everybody. When I was a kid, the music teacher in my town refused to teach me guitar because he said my hands were too small. I wouldn't be able to play the repertoire they were teaching. That stopped me from playing for many years, then I said you know what? I'll build my own repertoire. From that day on I made my own music. Music I could play and felt confortable in. I wish I could see that teacher again .... :cool:
So yes, look at what the "standards" are, they have been around for a reason. They are proven to work for many people but if it doesn't feel right, adapt them to your body.