It doesn't matter what "backup type" you select when backing up Exchange. The Exchange Agent will use it's own backup type. 

Use the built-in Exchange Backup Agent to backup Exchange. Read the Endpoint's documentation for more information on Exchange. The agent does a full backup the first time it runs, and then backs up only new exchange emails or objects (or tasks, contacts, etc). There is no need to do a first full and then switch to Incremental. Just start out with Incremental backup for Exchange.