I personally start using nutes as soon as my plants are at least 3 nodes tall. Some people use them from the start and others wait till they go to flower. It all really depends on what exactly you are using and what results you want back. As far as venting the closet, there is 100 different things you can do. My suggestions is to get crafty and use what you have as long as you think it will work. Growing is all about trying new things and finding out things that work directly for you.