Do you think this would be a good places for a beginner, or a solo backpacker? Is this, at least, a typically nonsketchy/sketchy person free zone? As I female I worry going to newer trails I don't know to be completely honest.

Absolutely!

Absolutely! That's a perfect place

I went there with 50 kindergarten students. The staff there is amazing and the strangers we met along the trail were very nice and friendly. Most really just want to get on with their hike so kept it brief and went on our ways. But they were still nice

Absolutely! Try Mt. Wilson. Very clean, and safe

There are so many trails in San Gabriel mountain, don't which one you are referring to. You can Google it to find out details.