Newbie O365 Question - Creating New Users

You will be losing a bit of functionality by not moving Exchange workload first. Most orgs go OneDrive or Exchange first, then go for the other things such as Share Point / Teams etc. as they go hand in hand and piggy back off Office 365 groups.

Here are some examples of features you can't use or get when keeping Exchange Online on Prem: https://docs.microsoft.com/en-us/MicrosoftTeams/exchange-teams-interact

Also before Teams you'll want to get OneDrive going since it integrates pretty heavy into Teams along with SharePoint: https://docs.microsoft.com/en-us/MicrosoftTeams/sharepoint-onedrive-interact

You could be asking for some routing headaches and Outlook client issues trying to load you Office 365 mailboxes if you enable your licenses. I would suggest you hold off on turning Exchange Online licenses on until you're ready to Migrate. Since you would be doing a cut over migration you would have to have some downtime and time to import all the mailboxes across for when you swap your domain routing over as well.

If you setup Hybrid you could do this a few users at a time.