Apples & Oranges?
I'm not terribly knowledgeable about the TOSlink technology, but I know that it sends a digital signal. This is obviously different than analog signals that run between older devices that use a Left and Right channel (usually white and red).
My response is simply to explain that the data sent between two devices using TOSlink connections are ignorant of how many channels they contain.
It would seem that the question to ask is "Does the sending device, the MacMini, and the receiving device, your pre-amp or receiver, handle five or seven discrete digital channels that it can convert (via D/A converters) to signals for five or seven speakers?"