There is an argument if having a mistress is okay and most rishonim allow this. but even the Rambam who does not hold it is an איסור עשה [prohibition derived from a positive command]. That is how the Beit Shmuel in Even Haezer explains the Rambam. For the Rambam does hold that a girl who is willing to have sex with anyone for money--i..e. a prostitute is forbidden, but  that is not the same thing as when one has a mistress or girlfriend who is only for him alone. But to most rishonim this is permitted outright.

The Gra makes note of Caleb ben yefuna in chronicles I ch 2 verses 43-50 who had a few wives and a few mistresses. And he was not a king. So the Rambam who says that a mistress was permitted only to kings is incorrect. 

As the issue of the monthly period, she should dip in the sea or a river