Company aquisition with another 365 tenant

Hello folks

Our business >1400 employees has bought out another business >500 employees as we are the group business we will want to onboard these users and data Sharepoint/Teams/OneDrive and Exchange mail + archives at somepoint this coming year to our tenant.

I have some questions around this:

1, What are the best practices for merging these 2 tenants, are there any guides or 3rd party SW that can assist with moves?

2, Some of the other businesses users have a need to access a shared mailbox on our current tenant, is this possible without having to create them a new user account on the main tenant as we are already paying for Lic's on the other side? can external guest access be somehow used?

Thanks for any input