Exchange 2013 to O365 Hybrid Deployment Issues/Questions

We are planning an Exchange 2013 hybrid deployment with Office 365.

Current infrastructure:
4 Exchange 2013 servers, 3 of them in a DAG, 1 Separate for another domain.
All mail servers are in my mail domain.

The issue I am having with research is our current deployment doesn't match any of the hybrid deployment guides.
Our AD Forest has 6 sub domains one of them being our mail domain where our mail servers reside.
The department that wants to go to the hybrid deployment is setup differently than all other departments.
They have their own forest/domain with a two way trust with the mail domain.
All of their accounts are on a separate server that is not part of the DAG and a separate database that is only on this one server.
All of their accounts are linked accounts to their separate forest/domain.
Our initial plan is to only migrate 35 users then get the other 900 moved once the deployment is successful.

Any advice is much appreciated
https://docs.microsoft.com/en-us/exchange/exchange-hybrid
Brandon MillerSystems AdministratorAsked:
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timgreen7077Exchange EngineerCommented:
Last i heard linked mailbox migrations are not supported. You may have success using a 3rd party tool or export the mailboxes to pst files if they are small enough and them import into the O365 maibox, but natively this was not a supported option, which explains why you can't find any documentation on that.
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Brandon MillerSystems AdministratorAuthor Commented:
That's what I was worried about so I guess they are going to have to take the all or nothing approach because I cant change the mx record to point to o365 for those 35 users. Sounds like they are going to have to get everyone moved at the same time.
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timgreen7077Exchange EngineerCommented:
Unfortunately.
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