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.

Latex paint works just fine.

Do NOT cut latex with acetone. You will not be happy with the results.

Use water it works just fine and It's cheap.

To paint your house maybe.