I have a 100% on-premise setup going currently, with Active Directory and Exchange. Windows 2012 R2 servers, and 2008 R2 servers.
We have O365 licenses that are not being used, unfortunately. I want to sync a pilot group to start using things like Skype for Business, and SharePoint online, then later down the road have a hybrid Exchange setup.
Couple issues I noticed, however, is there are a couple hundred accounts already sitting in O365, that appear to be the result of a screwy sync a while back. I don't think deleting them will do anything, as I know nobody in the organization is using O365 for anything (besides people like me with an Admin account that can login and manage stuff).
Is there any harm in removing all those accounts from O365, and re-deploying a DirSync server? Deploying the DirSync server, does that start automatically doing things an admin wouldn't want it to do? Or when deploying, am I setting it up on my terms? Like being able to only select a specific OU for syncing (like my pilot group)?
Lastly (I think), is there a need to verify our federated domain (we'd be authenticating with O365 with our UserPrincipalName) with an mx or txt record or anything, or is that only for Exchange Online?