AD Sync

The previous farm admin for the SharePoint 2010 farm I inherited had tried to run AD Sync unsuccessfully for some time. I noticed that the user profile sync service was not started and after working through the issues getting that started, I noticed that the Sync connection settings had populated the containers, but that they had checked one of the entire domains and everything underneath it for a sync. This doesn't seem right to me, we are a small org in a larger, much larger org in fact.

In any case, that brings me to my question: if I sync just the containers in AD that I think we actually need, it will merely overwrite any of that existing information in the user profile database, correct? If there is other information that is existing in the database but I'm not syncing it, it stays the same. Basically, what I'm getting at is that I want to make sure when I sync this, if it turns out to be the wrong container, it doesn't do to much damage, at least until I can do another sync with the original settings.