Adding a new domain (company acquisition) to existing Office 365 Environment

Yes, you should enable Exchange services to proceed otherwise the domain will never go onto Exchange Online and you will not be able to do steps 2 and 3.

If you are unsure about the migration route to take, I would advise not to proceed with adding the domain until you have a clear migration methodology. As Vasil said, it is only 200 users so this usually fits a cutover migration. You can check out the ways to migrate to Office 365 here

https://docs.microsoft.com/en-us/exchange/mailbox-migration/office-365-migration-best-practices

https://docs.microsoft.com/en-us/exchange/mailbox-migration/mailbox-migration

And follow this guidance.

Best, Chris