I understand that skim milk is available on all Princess ships. If you don't see it, just ask.
I'm not sure about 2% milk, but you can always make 2% milk by mixing equal portions skim and regular since regular milk is 4%.
You probably can get chocolate milk in the dining room since they have chocolate sauce available for sundaes.
Anybody who needs the soy substitute should advise Princess in the "special dietary requirements" section of the information forms. Princess will provide such things if given sufficient notice to arrange their delivery to the ship. It may be in the ships' standad inventory, but it's better to be safe than sorry.