Can only answer a few questions.
-Staying in accommodation when under 18 is recommended however I do know a few under 18's who haven't stayed in halls. If you do not wish to stay in halls, you are required to write in person to your training provider explaining why.
-Assuming you're with CMT, you will be issued with a NATO jumper, epaulettes, along with some shirts, however this is not usually until around a week or so into the course.
This is just based on my experience, and things may be slightly different now!