Although the word Hijab isn't used in the Holy Qur'an, there is an ayah in the Noble Quran that calls for the believing woman to let down their head coverings over their chests... I will look for it for you.

And yes, the prophet (pbuhahp) had more than one wife. But he did not marry any wives until after his first wife, Kadijah, peace be upon her, died (so he did remain monogomous for... ~20 years?). When he started marrying other women, he married them primarily to take care of them because either they were widows of his martryed companions, or women of other tribes (which helped reconcile relations between Muslims and other tribes. Although the Holy Qur'an gives many instructions to fight the infidels, the Most Gracious and Merciful Allah always commanded Muhammad, peace be upon him and his progeny, to accept peace whenever the unbelievers would offer it, even if there was a chance of being deceived by them) and I know in one case he married one woman in order to dispel the superstitious belief that you cannot marry the wife of one of your adopted sons (Just because you say "this is my son", he is not your son, and just because you say "this is my mother, she is not your mother" - this was directed to the superstitious arabs).

But by the time he was marrying and caring for these women, he was far from the standard "age of vigour" for young men. He never married for the sake of lust and satisfaction, although some non-Muslims might assume that he established polygamy for the lust of men when they hear about the prophet's (peace be upon him) wives.

I know when I first heard that he had married more than 4 wives (the limit of all Muslim men), I thought that this was a sign of curruption in Islam. Of course Allah, subhana wata Allah revealed a verse in the Qur'an that very clearly excuses the prophet (pbuhahp) and only the prophet from this limit of 4 wives, but the same ayah directs him to not marry any more wives after the ones he already had. The prophet (pbuhahp) never married after that ayah was revealed, but he was allowed to retain the number of wives that exceeded the four, and other Muslims had to release theirs.

Surely Allah (SWT) knows best the limits of men, He is closer to us than our juggular vein, and surely He comprehends the needs of the world.

Asalamualaikum