-  If an Ordnance rating (Gunners Mate or Fire Controlman (now ET-weapons), you could be an E4 to go to SAI school.  If you were another rating, you had to be an E5.

-  Almost any rate can be an SAI (back then...  this may have changed).
     There was a Cook, a Telephone Technician, a Mechanical Technician (MK) and several Boatswains Mates in my class.  There were more Gunners Mates than any others.

-  Command Recommendation / Need.  Your command must have an open billet or have a demonstrated need for an SAI in your area.  Keep in mind, when you are an SAI, you are NOT the personal SAI for your UNIT.  You are now a DISTRICT Asset.  I believe Districts are now called Sectors or something else.  Maybe Groups are now Sectors...  I don't know...

When I went, I had gone to GM "A" School then been assigned to TRACEN CAPE MAY as a range instructor.  Back then, recruits shot real pistols and rifles.  We also provided training for all local cutters and small boat stations within an hour's drive from Cape May.  Some units came farther if they needed to get a few people qualified to go to a school or deployment on short notice.  I was not an instructor for anyone else but the Coast Guard.  On my own time, I did teach friends, family and others how to shoot a few times, but not for pay.  I had several friends recommend I get NRA certification so I could teach outside the CG, but I never did.  I guess I still could, but my current health prevents that.  

Don't worry about being a non-rate for a while.  Waiting for your school will be well-worth the wait.  You will learn a lot about the CG as a non-rate.  I would recommend you ask for a cutter after boot camp.  The bigger, the better.  White is preferable to Red or Black, but those missions are fun, too.  White ones go more places and have more chances of doing stuff you like.  If you're still waiting for school after the end of your sea tour, see if you can get a small boat station.  I had fun there, too.

Any other questions, I'm here...

Mike