So here's what you'd do:
1) Create an auto-playlist on MMW (Gold) that contains e.g. 20 most recently added tracks
2) Enable the sync option to Delete tracks not on the Sync list

Next time you auto-sync it'll sync the 20 most recently synced tracks, and delete all other media content from folders that are being synced on the device.

Then if you add 7 new tracks to your library and resync
--> 7 tracks from the previous sync will be deleted, 13 will remain on the device, and the 7 newest tracks will be synced

-Rusty

Yes and no, Auto-Playlists are capable for what you want, but they are handled on MMW (Desktop) side and after Sync they are Static on MMA.

NOTE: You would need to create Multiple Playlists eg. AN than use one playlist that will sync results from other playlists. It is bit touchy, but possible.

Mediamonkey has smart playlists on the desktop version basic auto list in the free version ,and advanced autolists in the gold version.
Im not sure but i believe you are or are going to be able to load those autolists in to mma, which will or should make it possible to do this.

I use an app called "beyond pod" to manage my podcasts on my phone. A great feature about this app is "smart playlists" which queues up a specified amount of podcasts from a specified playlist. ie play the 2 newest episodes from the "news" category followed by the 2 newest episodes of sports, followed by 2 newest entertainment..... etc. Mediamonkey could use this same feature by creating a smart playlist like: play 20 songs from playlist "new songs, followed by 20 songs from playlist "favorite songs" etc.