I came across a unique situation. Company A is in the middle of the child forest consolidation. Company A has multiple child forests an account forest and a server forest. They are in the process of consolidating into the root forest for a single forest. They are also in the process of a merger. If accounts are created in the root forest for Company B that was acquired, can users be added to Exchange Server in the child forest.
Another side. We are considering putting the user accounts in the root forest using azure AD Connect in the child domain to sync them to Office 365. Is it also possible to add the users to local exchange as remote mailboxes in Office 365 if the AD account sits in the root domain?
You should know going in however, linked accounts make a migration to O365 a bit more interesting when you get into it, and typically we just would commit and migrate that user to the exchange solution in full, prior to a move to O365. (so would move those users twice, once to the AD/forest for the one prem exchange, and once to the cloud).
In O365, your AADConnect can only have 1 server per tenant. So to your question, if your AADC is running on your root domain and syncing up to O365, then any accounts there will be fine.
Big picture though, if this was my project 1000% i would focus on getting the identity cleaned up and consolidated. Identity is the foundation that you build your organizations house on for O365 and the cloud. If it is a shaking/messy foundation it will plague you with problems down the road. Sometimes timelines force a temporary situation that is a bit messy, but work towards the end goal of having one consolidated AD if possible that runs up to O365.