Should I rent a room to by best friend and her boyfriend?

My fianc� and I are getting a 2 bedroom apartment next week here in San Diego. My best friend who moved to Florida last year wants to move back and asked if she and her boyfriend of 8 months could rent a room from us. I have a few concerns, however. One is that she isn't as financially responsible as I am and I am afraid she won't pay the rent... Our mutual friend bre says her boyfriend is financially stable and that she doesn't think that would happen. My other concern is that it will be awkward having herb around my fianc� and me around her boyfriend so much. All of us are around 20 and I am worried that something bad could happen. My fianc� has given me no reason not to trust him I just male trust issues because of my dad. Am I being unreasonable? My friend isn't a flirt but she is really cute. I am too but she is also two years older and I'm afraid my fianc� will start to like her because she is so pretty and nice! But I can also see her living there as being a lot of fun..what to do?