If I go into the Army as a prior service Marine, is it possible to go in at a lower rank?

There's no reason for you to lose your rank coming into the Army as long as you've completed the Marine Sergeants Course. That's the equivalent of our Basic Leaders Course. Everything you need to know about the technical aspects of running an Infantry fire team as a SGT you can learn in a few weeks at Basic or even Ranger School. Everything you need to know about being a Leader will come back to you quickly from your previous experience as a Sergeant. The terms are different but the people are the same. We call them Privets you call them Boots. They're just as ugly and just as dumb in every service, and they're all trying to buy a Mustang at 28% interest and marry a stripper. That's the real issue. The technical stuff you'll pick up after a few field problems and there is a manual or regulation on every single thing you do from shooting optics to tourniquets to answer any questions you may want to brush up on.

You might find that your level of responsibility as a Marine Sergeant was higher than what it will be as an Army Sergeant.

If you go in as 11x you'll make E-5 instantly, even if you were reduced in rank. Promotion points are always low. Recommend just doing it as an E-5. MCT was more difficult than 11b, honestly. Only difference is they go over flanking, so you'd definitely have more experience overall. Send it