For bgs, stripped down to new metal with a light surface rust. Should I use rusty metal primer or enamel from rustoleum? Does it matter? Longevity wise?

I have used both as well as mixed as I ran low. They both work well. The key to a good paint dip is thinning. I only use acetone and run a ratio of 40% paint to 60% acetone and have had good luck with even thinner paint.
I have tried paint thinner and had poor results. Use acetone with the above ratio and you will be happy. And be sure to mix well.

I do 2:1 acetone to paint for rusty metal primer and than 1.5:1 ratio for flat black on footholds.

Thats starting from more or less bare metal.

I think the primer is important for “dirty” bare metal.