backbone cable type

You could. Fiber will allow for more growth in the network though, so you should consider how much larger your network will grow and how quickly that growth will occur.

There is nothing that says you can't. However, you will have to work within the limitations of UTP, i.e.
1. 100m cable segment length between network devices capable of retransmitting/amplifying data signals and/or end nodes.
2. Subject to EMI (electro-magnetic interference), which will dictate where you can or can't run backbone cabling.

For these reasons (and the reasons mentioned in Answer 1), unless you have a very small backbone you wish to construct, you are usually better off with fiber.

The question I have to ask is why. Do you have a closet full of utp cable and connectors that you need to use.

You will have fewer headaches if you use cat-5

Good Luck Lee