I have a WP 7.5 Focus phone and Zune running on 2 Win7 PCs, and I'm on my third joyous year with a Zune/XBox Pass. On your second question, your WP syncing will/should not be affected in any way. Adding an RT to my account last November did not affect my PC nor phone-syncing in any way.
On your first question, the tricky thing I found about the RT is that it doesn't seem to sync with anything except the servers; that is, with your Pass music, and not with your phone, nor even with your PC's Collection, which includes your Pass music, plus any personal music you have ripped or otherwise added to your PC/Collection. Also, note that from the moment you set up the RT, you are signed on to your WinLive/Pass account. Thus, unlike the PC, the RT is always connected (through apps) to your cloud-based everything -music, SkyDrive, MarketPlace, etc. As I recall, what I did initially was to load all of my PC's My Music folder onto a USB drive and then drag the music to my RT. I had all sorts of problems, early on, playing music on the RT, with rights and XBox-shut-down issues, among others. Then. at some point, I just deleted all the music I initially transferred -using WinExplorer on the RT. Then I shut down or restarted, and the next time I ran the Music app, I waited and after a few minutes the 4000+ Pass tracks (purchased and not) started downloading to the RT, with a small unintrusive message in the upper-right-hand corner alerting me and counting down. This took a while, but after that, I had to transfer my non-Pass music (only) via the USB, and my rights and shut-down issues went away.
But, again, unless there's something I haven't figured out, the RT will not/cannot sync in any direct way with your Collection, and you will have to continue to add or delete non-pass music. Also, the syncing with my PC Collection isn't as immediate and logical as it is with my phone or Zune HDs. Collection deletions, for intance, are not always deleted -or deleted right away- as they are with my other devices.
Enjoy your new RT. It's a new device with new software, and the RT, in particular, is unlike most other devices. So, as early adopters, we need to be patient and curious to learn and enjoy --and sometimes to share, and ocassionally to teach.
Of course, I hope others will help all by correcting, qualfying, or adding to my notes/recollections. It's also possible that RT or app updates make my early experiences outdated.