Is this a belief held by all types of Muslims though? Some of the reprehensible beliefs attributed to Muslims end up only belonging to certain groups of Muslims.

As I understand it not all Muslims even accept Sharia Law which is what sets a lot of these restrictive rules. Is separate swimming something all Muslims want or only a subset of them?

I doubt that in a group of one billion people you will find total unananimity of opinion about very many topics of their religious interpretation (especially those as arcane as when and under what conditions a female may expose her face to others) of someone else'e interpretation of some book(s) some guys wrote hundreds or thousands of years ago about what "god" "inspired" them to believe. The point is that a country that has no past history at all of such beliefs (including not shaking hands with women, etc.) is altering the laws of their country in order to accommodate whatever-the-number-is of foreigners who refuse to assimilate into the culture they chose to emigrate to. How many "infidel" head-loppers does it take to spoil your day? Do the math... say one-half of one percent (.5%) of a billion...
Whether the "subset" is one percent or ninety-nine percent, Sweden is changing their laws to accommodate that extremely small group of people. You should watch the BBC documentary about "Dating in Iran". The woman reporter from the UK was "assigned" an "interpreter" by the government (be sure to note the "interpreters" comments while lecturing a college class on the immorality of American women for dating several men before marriage), who accompanied her everywhere she went. She mentions once during the program that she could not ask questions about numerous topics (sex, etc.) because, well, it's Iran, they're muslims, and that's not allowed. What's obvious throughout the entire program is what is not being said: every single move by all these people is being monitored by the government to be in accord with the laws of the theocracy and there is no room for nor tolerance of those of other "views". Not a single person in the whole program is anything other than a serious devotee of islam, and obviously, no homosexuals are looking for partners in Iran, because there are no homosexuals in Iran (though a cynic might argue that there are possibly a couple of gay people in Iran who are "in the closet" due to their fear of being thrown off a tall building, as prescribed by muslim holy literature). Allahu akbar!! The phrase Allahu Akbar is written on the Iranian flag, as called for by Article 18 of Iran's constitution. The phrase appears 22 times on the flag. 

Interesting, I had to look at the Iranian flag to check that one out!

I was actually interested in what percentage of muslims held these beliefs, not whether they were justified in modifying the culture for them.

In all the discussions people have about muslims and islam it ends up that people often have misconceptions and misunderstandings.

How would you find that out? Are you going to submit a required questionnaire to a billion people and assume they all return it and answer honestly? Do you think any poll of a group of muslims will come up with an accurate percentage of honest answers to questions like "Do you believe that infidels [nonbelievers] ought to be subject to random violence and beheading as prescribed by the prophet? allahu akbar!"? No one knows what that percentage is regarding any issue in islam, that's why I said above, even if it's less than 1/2 of 1 percent, when you apply that small percentage to a billion muslims, you get a shitload of trouble. You might as well ask Hawaii gun owners "Have you ever violated any Hawaii law regarding firearms (e.g. stopped at the gas station on the way to or from the range, etc.)? Who's gonna answer "Yes. I've violated the law"?
The muslims or the nonmuslims?