Your instincts are already pointed in the right direction. The middle ground between too friendly and too hard is exactly where you want to land, and your background actually gives you an advantage in finding it.
With a one to two year sentence you are most likely looking at a lower custody facility, possibly a camp or a low-security women's institution. The population at that level is generally not looking to create serious problems. Most women in that environment are close enough to the door that catching a new charge or an incident report is the last thing they want. That reality keeps the temperature lower than people expect going in.
The marine in you is an asset. You already know how to project calm confidence without aggression, how to follow structure, and how to read a room. Use all of that. Walk with your head up, make eye contact when you speak to someone, and keep your voice even. Confidence that does not look like a challenge is the right register.
Keep your personal business private. Do not talk about your charge, your case, your co-defendants, or your life on the outside beyond the basics. The less information you put out there the less anyone has to work with.
Be cordial without being open. You can be polite and decent without being someone's best friend in the first week. Let relationships develop naturally over time with people who earn your trust.
If something physical comes at you, handle it. Letting something go unchallenged in that environment sends the wrong message and invites more. But do not go looking for it and do not escalate situations that can be walked away from.
A year or two goes by. Keep your head down, stay busy, and protect your release date.