I think six parts is a good target for most things. I've had a couple of times when I can't get below seven, and I have a long-term project in mind where.. well, we'll need every body that can blow a horn that we can find. ;-)
The jazz I'm finding is usually heavy on piano (naturally... one of the most common trio arrangements is piano, bass, and drums), which means that it isn't going to work well. Sometimes I can mix down to a decent lute/harp track from the piano... a lot of the time it just gets to be mush. I do have quite a few quartet pieces, though.
For Rock, I've got a few things in my dropbox folders (some may be more pop, but some are real old Rock greats, like the 3 Dog Night stuff)...
In NEW_Testing: Set Fire to the Rain, Famous Last Words,Grenade, Heart Shaped Box, Mama Told Me Not to Come, Old Apartment, Old Fashioned Love Song, One is the Lonliest Number, RockSteady.
In Working: Passage to Bangkok, Tom Sawyer.
In Pop_Rock: Final Countdown, You Can Leave Your Hat On, Right Now, (It's Raining Men is more pop, I think... and probably Novelty is a better folder for it. I play a solo version under a jumping puzzle in Rohan where people fall constantly.)