Your description of military nursing seems Navy-esq...or just really old-school. From the Navy nurses I've talked to, it sounds like everyone can move around from ER to ICU to PACU with local, homegrown, on-the-job training and the blessing of their leadership. The Army and AF don't like homegrown training.
"Starting in the ICU on day 1" actually is how the Air Force and Army work, assuming you come in with the requisite civilian ICU experience (often 1 year). It's actually listed on your contract.
As a currently active duty AF nurse, I can tell you that the AF is very protective and rigid about their AFSCs (specialty codes). If you come in with an ICU code, then you will 100% be working in an ICU for your first assignment. Our codes are coveted and difficult to get, and you can't just float down to the ER one day, work with a preceptor for a few weeks and become an ER nurse.
The AF sends nurses that want to specialize beyond outpatient clinics or med-surg, but don't have any experience, to formal schools
. These can be anywhere from 6 weeks long (inpatient psych) to 1 year/ long (ER and ICU).