From which level of education, is it better to choose teacher assistants? I think that it is normal to choose teacher assistants for BSc courses from MSc or PhD students and TAs for MSc courses to be from PhD students;  

In case that the PhD or MSc students are not interested or are not applying for a teaching assistant position of the lower courses (lets say a BSc course); is it a good idea to choose an assistant from the interested last year BSc students (with good academic records and background) or it is better not to have TA for the course for a period of time until a PhD student applies for the TA position?
Some universities have rules about what kind of student may be a TA.  Assuming no such rules exist, the TA should be qualified to fill that role, whatever that means for a given course.  That is, the TA should have both the domain knowledge and pedagogical ability that the job requires.

It doesn't matter what level of education the TA is at. In my own department it is quite common for TAs to be "peers" of the students in the class (B.S. students hired to TA the B.S. level courses, M.S. students and PhD students hired to TA graduate courses including both M.S. and PhD students.)