It is possible to enroll in a magistracy in a specialty other than bachelor's degree. Before submitting documents, we recommend that you write to the Admissions Office and inform you that you intend to study for a master's degree in your chosen direction and clarify whether you need to take prerequisites. Prerequisites are disciplines that must be studied before starting a master's degree. Sometimes an American university asks for a candidate's transcript and tells you which disciplines you need to pass, and asks you to provide a document confirming that you have studied them. Some American universities answer that there is no need to study any additional disciplines, because all required courses will be completed within the Master's program.
Yes. There is no statistics at hand, but it seems to me that in the masters program, for example, in statistics, about half of the students received bachelor's in a different, but similar specialty - applied mathematics, computer science, even economics or physics.