As long as you don’t shrink below 70%, you won’t see any difference. The more you shrink, the less the quality, but that’s true of any program you’ll find.
Although some consider it too antiquated now, I find DVD Shrink to have a versatility that most of the newer programs don’t.
I personally run AnyDVD with it, and sometimes use CloneDVD2.
As for putting files from several DVDs on it, save the discs as ISO files (using something like DVD Decrypter or similar) to your hard drive. Then, using DVDShrink, go to File -> Open Disc Image -> and drag the file that you want over to the left. When you’re done with that file, go back to File and do it all over again. There are excellent guides out there - Scuba Pete, Mr. Bass et al…
There are lots of proggies out there, but this one’s still pretty good! And it’s free. That’s always nice!