o yea!
you paid for it and you can ask for changes
within reason
if they are coloring a picture dont ask for a new pose
but if they are finishing the skeleton or want small details
go ahead and ask
i know when someone commissioned me i asked every 5 minutes if it still looked good

A person got all mad cuz I asked for the shoulder to be less angular.

well that person has no right to get mad!
you paid for art and asked that the artist make a small change
there is no problem with that at all hun!

Welcome to the world of commissioning gal
Ask your friends who has a good rep and who doesn't if you like an artist just note one of their other commissioners and ask how they act
I've done it many times before and has saved me from some bad experiences.

NOW it's understandable for an artist to get frustrated if you are trying to like be a back seat driver and nitpick over a lot of stuff. But at the same time it's their responsibility to get it right and if they feel like they can't do what you want it's their responsibility to deny your commission and if they do to not take it as a personal offense.

I don't like taking sides after hearing only one person's perspective, because it wouldn't be fair for me to do, but all I can say is to be patient and let them be a little frustrated. That's not to say that they have any right to take that frustration out on you, of course, but you can't deny someone's right to have emotions. All I can say is that they, as should you, are expected to be professional when the subject of money comes into the equation.