the barrier question. Here is the test you should perform. Get some plastic, visquene or plastic drop cloth. Spread it out on a small area maybe 3 foot square. Leave it there for a day or two. Then pick it up and if there is condensation, water vapor, or any moisture, you need to address that first. This has to be fixed with a sealer or a barrier. You have got to stop the moisture from seeking the wood.
these screeds will work, but the closer together the better. Glue and screw them down. Do all the homework before laying the hardwood so that you do not have a bounce, a sweak, or any movement. Use galv or zinc coated long screws and go into the studs or joists. If this is concrete, then invest in a good Remington 22 concrete driver and attach that sucker good.