I personally don't think a barrier coat should be done unless:
1. you have a blister problem and the barrier coat is part of the solution.
2. you have a flaky bottom due to excessive paint buildup and you intend to strip the bottom anyway to start over. (In this case a barrier coat becomes a convenient precaution.)
Stripping otherwise good paint off a sound bottom to barrier coat it is a waste of time, money and paint. This is especially true if you have been keeping your boat in the water for most or all of the year. If the bottom has been wet for a few years or more and does not have blisters, she doesn't NEED a barrier coat.
Ditto the Micron Extra recommendation, you can find compatibility info at http://www.yachtpaint.com/USA/hotlinks/compat.pdf
and if you have questions call their 800 number, 800-468-7589 they are very helpful on the phone.
PS - FWIW you only need to add ablative paint when the existing paint is close to gone, so try to figure out what paint is left on the bottom - you may not need to paint at all. If you just add more paint when not needed, you eventually end up with a heavy buildup that starts to flake and REQUIRES stripping to remove.