You can sync multiple AD Forests to a single Azure AD/Office 365 tenant. You'll need to use the custom installation option with Azure AD Connect.
A Forest Trust isn't required. You just need to have DNS name resolution between the Forests, which you can achieve with Conditional Forwarders.
The easiest way to filter user and group objects is to set the
adminDescription attribute to
User_NoAzureADSync to filter users
Group_NoAzureADSync to filter groups
You can use the Azure AD Connect Rules Editor for more advanced filtering, but TBH, you probably don't need that.
Note that any object where the
isCriticalSystemObject attribute is set to
TRUE won't sync to Azure AD/Office 365.