When going SF, can you choose your MOS?

No. MOS is based on your aptitude’s, gleaned from your prior MOS, how you perform in SFAS, and the needs of the Regiment. They’ll ask what you’re interested in, but that’s just a small part of the consideration. If you ask for 18D Medic, and your GT/test scores are good, you’ll probably get it.

National Guard you have a little more freedom to get what you want because it's based on the slots in that unit. Big army SF, you give your top 3 and they tell you what your MOS will be.

You cannot choose your destiny. You can only choose to accept it.