I work in LCPS. The only elementary paras past kindergarten in gen ed are for IEP students. These jobs are actually pretty coveted, but you can't choose your grade and will almost certainly be assigned to students in multiple grades.

It depends on so many factors. SPED or gen ed? You may be assigned a specific class, a specific grade level or multiple teachers/students. It can change year to year. It could be push in or push out. I have done para for elementary and finally moved on to middle. I LOVE middle school. The students are a lot more independent, you can reason with them a bit more, can truly see their progress and the behaviors are bit less than elementary.

It's really dependent on what age of kids you connect best with. I did various grades but looooved Kindergarteners. Others excel with the older kids. I'd bounce around and find your own sweet spot.

Aside from kindergarten assistants, I have never known them to be up to the person. Admin assigns them to grade levels (either special education paras or those hired for overfull classrooms) at will.

I'm a teacher and I'd say around 3rd. No higher than 4th and no lower than 1st.

It depends what kind of para job you want. A lot of people will tell you 2nd-4th grades is kind of the sweet spot. Younger than that some students may not have had a lot of time in a classroom and require almost constant 1:1 assistance with everything from using the bathroom to using the playground equipment safely. In any K-2 class there are always a couple kids who haven't been identified for services yet even though they're clearly in need and that can take up a lot of your time even if you aren't assigned to those students. 5th or 6th grades means the kids are bigger and stronger. That might not matter too much if you aren't dealing with students who need a lot of behavior support.

What grade is the "best" to be an instructional assistant? Kindergarten? 3rd? 5th? (elementary schools)