I would probably get them established eating rats first, but it wouldn't hurt to offer a f/t rat. Worst that happens is they refuse and you give them live instead.
A rat fuzzy (or smaller) is perfectly safe to leave with your snake unsupervised, but once they get much larger than that (pups/weaned and larger) they can potentially cause harm if you aren't feeding responsibly. Feeding live isn't as dangerous as some people (namely folks who have been ill informed by pet store employees, etc) will have you believe. Most of the horror stories you'll hear about are from irresponsible owners who leave rodents in with their snakes for extended periods of time unsupervised (sometimes days at a time!). Feeding live is all about using common sense and being responsible. Never leave a rat larger than a fuzzy alone with your snake and supervise all such feedings. I like to keep a pair of tongs handy to put in the rats mouth in case the snake makes a bad strike. Also be prepared to have to thump the rat (ie knock it out with a whack to the noggin) in the case of a really bad strike. Sometimes you might need to help them by readjusting their coils as well. If they don't seem interested in eating within a few minutes of offering I remove the rat and wait til next feeding. I have fed thousands of live rats over the years and I have never had one serious injury caused by feeding live.