I just thought I'd update here in case it helps anyone. I called my insurer (Geico), and they confirmed that a rental car would be covered for me. They also confirmed that if I rent the car, as long as the bf is listed as a driver on the rental car agreement, then HE is covered by my insurance too as he doesn't have his own policy. Without adding him as a driver to my policy. They said I don't need to add him to my policy as he doesn't drive my car more often than once a month. [I did say he was my fiance, btw.] I am a bit skeptical, to be honest. But we'll take the chance and do it the way they suggested.
In other news, I managed to lower the cost of my policy too, as doing all of this, including getting a quote for adding him anyway, I realized that my driver information was out of date and that Geico had me down as driving a lot more than I currently do. Now that I'm back down to under-average amounts of driving, and now I've told them that I'm driving less, it is going to save me money.