Like John says, it is safe to use a fast acting fuse in place of a slow blow. It just might fail where the slow blow would survive when you get close to its limits. Replacing a fast acting fuse with a slow blow usually isn't a good idea, however.
A fast acting fuse doesn't actually blow as soons as its current rating is reached. If you were to look at the ratings of a garden variety fast acting fuse, you might see ratings of a few hours at 100% current, a few seconds at 200% current, and a curve in between. Slow blow fuses just give you a little extra leeway over fast acting fuses to handle starting current, like charging power supply capacitors.