What glue do you use?
I am going to float engineered flooring, and also staple/nail down in another part of the house.
The engineered wood is 3/4 thick, laying over OSB where nailing.
The manufacturer recommended I use 18 gauge, 1.5 inch long, narrow top staples. One problem is that I cannot find a stapler that fits that bill, and states it can handle thicker than 5/8 inch flooring. What would you do?
What underlayment should I use if I want to keep the sound down where I am nailing the floor down. What would you use if you were putting this in your house?

So. Staples or cleats?
What would you do about the stapler for the recommended staples?
What glue do you use?
What underlayment would you use?

Thank you.

I would use the regular 15 gauge staples used for solid flooring. It will not hurt the edge where the tongue is. You can order the one and a half inch length.
If you want to keep the sound down, then use soft soled shoes or slippers.
Poly-urethane glue for hardwood flooring will remain flexible and will hold the best. Be careful to wipe any spots of glue off the pre-finished flooring immediately or it may etch the surface I like to use Bostik latex sub-floor adhesive for starting and finishing rows since it is easy to clean up, costs very little($4 for the large tube), and stays flexible, but holds well.

I would suggest using paper underlayment like Aquabar-B or Silicone Vapor Sheild for nail-down hardwood. Simply roll out the felt paper over the OSB subfloor panels and overlap the seams about 4 to 6 inches. Use ordinary 15 gauge staplers to attach them to the subfloor.

If you're choosing to glue down your hardwood, water based adhesives are a good option. Unlike urethane based glues that offer very little corrective measures, water based glues are strong and add a weight to the plank when dried. They are also easy to clean. Simply use mineral spirits to dissolve any extra paint or varnish, wipe using a mild detergent, and finally rinse in clean water.