Is it necessary to take all da xams in one sitting? I even heard that if we take the subjects of science, we won't be able to take the commerce subjects. Is that true? I am really worried about this. Different people say different things to me. I am not sure. Please help.
No, it is NOT necessary to take all the subjects in one sitting. There are some subjects with which you cannot appear in some of the other subjects in the same sitting.
For example, if you are giving Chemistry (5070), you cannot appear for Combined Science (5129) or Additional Combined Science (5130) in the same sitting. For such subjects , check the first page of the syllabuses.
As far as your question about giving Commerce Subjects with Science Subjects is concerned, you should check your school policy about that. Different schools offer different subjects. But in most of the schools you won't be able to take Commerce Subjects with Science Subjects until unless they offer extra classes. Otherwise you can give Commerce Subjects with Science Subjects privately.