Assuming you're not breaking copyright law, MPEG Streamclip (http://www.alfanet.it/squared5/mpegstreamclip.html) does conversions from DVD to FCP-compatible QuickTime movies, but when I tried it on a commercially purchased DVD just as a test, it was unable to break through the security. I've heard recently on here that Mac the Ripper (http://www.versiontracker.com/dyn/moreinfo/macosx/22715) can solve that problem.
Copyright law does not allow you to use non-licensed material for commercial purposes. Theoretically speaking, you can't even use a copyrighted movie clip or music on, say, your own website, nor can you make a copy of a CD or DVD even for your own backup -- "fair use" is something of a misunderstood term. You're not likely to be sued if you're just making your own music mixes for listening as long as you don't sell them for profit, but the law on paper does cover even that.