Isolation exercises have their place. That makes it sound like they should never be done. They should be...and they absolutely do "tax the muscle fibers enough to cause the muscle growing effect" when done correctly. The point is to target a specific muscle.

Dumbbells are great...I don't know why you would only want to train with barbells.

Mixing up repetition ranges can be a great thing. I believe it is called for specifically in the HST Training program listed in the Training Forum.