They're allowed, as long as they're notable enough.
So it's pretty open to interpretation, but basically a home-made compilation that you've given to a couple of friends doesn't really count, but a compilation you've made that has thousands of downloads might* be.
A live bootleg is usually notable enough, a live bootleg where you've cut up into tracks for personal use isn't, and so on.
Just make sure to mark the bootlegs as such, and please provide as much information as possible! A site that you downloaded them from or similar in the edit notes is really helpful, or even just a note saying you don't know where you got it from, so that we don't end up with a million unverifiable bootlegs.
If you have any other questions, just ask!
*in the end people will vote on your edits if they have an opinon, and it'll be worked out through general consensus. So don't be too scared of just going ahead and entering some edits, and link to them here if you want others to have a look