I'm assuming it is the caliper slide pin bolts you are talking about as they are the bolts that go into the slide pins. They should never have been that tight, some idiot probably drowned them in red locktite or something. You can drive around without the rubber dust boot on... but it will **** up the slide pin getting all the road crud in it, I would only drive it like that for a real short time and only if you have to. You can get those boots online for next to nothing or autozone would probably have them honestly.
2000 Mustang GT Steeda build #48. Bolt ons, suspension, Novi 2000.
Just because I give you advice, doesn't mean I know more than you. Its just means I've done more stupid ****.