Is it acceptable to ask a potential supervisor about my suitability for a PhD with them? Is it considered acceptable, in general, to ask the potential PhD supervisor whether one's profile is considered good/suitable for a proposed PhD (sciences) topic?

Countries: USA, Spain, Switzerland, France
Why not? 

Ideally, write them a short intro letter introducing yourself and telling them something about your background and interests and whether it would be worthwhile applying. Worst case, you don't hear back. Second worst, they tell you thanks, but it's not relevant. Third worst, they are interested, but have no funding. 

Make sure you check their website for their interests and research and ideally identify one or more papers which resonate with your intended direction and introduction. Some applicants manage to hit just upon something that the supervisor may just be interested in doing but never found someone to cover.