Previous Exchange Servers Co-existance with Exchange 2019
Exchange 2010 or Earlier is not supported.
Exchange 2013 with CU21 or above is supported
Exchange 2016 CU11 or above is supported
Hybrid Deployment Exchange 2019 & Exchange Online/O365
Exchange server 2019 supports hybrid deployment with Office 365 / Exchange Online
Active Directory Server Architecture for Exchange Server 2019
For best practice and security measures, it is strongly recommended to install exchange server on a separate member server and not on active directory server. Installation on a 64-bit hardware with 64-bit version of windows servers will increase performance and utilisation of full hardware resources.