Which container to use for combined milk?
Don't reuse an old milk jug for storing the new, combined milk. The standard plastics used in milk jugs in most areas cannot be properly sanitized for reuse (even with standard sanitizing solutions). 

Instead, use a properly sanitized and covered glass or plastic container, preferably one that you clean in a hot water dish washer.  Milk stored in a properly cleaned container will have the maximum shelf life (and reduced chance of other contaminants).