I have a client with about 75 mailboxes and plan to grow to 100 in the next year.
With our exchange rate, the licenses are very expensive but they want at least a few for some job descriptions that need calendars with coloured appointments, and therefore MS-Outlook as ZWC nor ZDC support coloured appointments and only colour a small bar to the left of the appointment.

Would it be possible to install Zimbra with 100 mailboxes, but only buy, say 25 licenses for using ZOC?
Or then, to have 2 Zimbra servers, one licensed and one not, but sharing mailboxes on the same domain and sharing mailboxes, calendars, contacts, tasks and briefcases between users on either servers?

Thanks for reading up to here... :-)