I normally press "Play" on the top half of the screen in a folder, which I think tracks what you're saying. I can certainly just select a track on the lower half of the screen, and will start to do so.
It sounds like "Category" is essentially the same as a file folder/sub-directory — is that correct? Is there a setting to turn off the automated play and shuffle at the top of the screen so Poweramp never shuffles/repeats without being deliberately asked to do so?