Hardwood Floor install Experience?

So when we bought our project house a couple of years ago, there were lots of things to fix and replace, clean up, maintain, etc...   One of those things is hardwood flooring in the main areas.    Anyone experienced in this area?     I laid 600 SF of Brazilian Koa in my last house and was really happy with the result.   This house should be a bit more challenging as we are putting in 1700 sf, with a set of stairs included.    I had a question I thought you guys might have an opinion on... underlayment:    The wood product is 3/4x3-5/8 short leaf Acacia. it's a nail down install over plywood sub floor.   I used 15# felt the last time I installed flooring, and have used Aquabar B in the past under tile sub floors but I have been reading about felt pads and cork pad to help dampen floor noise.    Since my install is nail down, I am worried that the felt/cork will compress when walked on, and will cause the cleats to work their way out, and the floor will squeak.    Is there any advantage to the thicker underlay pads?   

Thanks Pete - easier than I thought then.   I won't over engineer this, just straight up felt and I'll be good to go.

Kris,
As long as you aren't splitting the tongues, I see no reason to go with 18ga. Was the concern splitting or something else?