"From what you are saying, I conclude that inserting a 2.0 device is actually a good thing, or at the very least I should not expect it to cause any difficulty."
- No, ideally you be running as little devices as possible directly on the device that is responsible for generating your audio stream. However, if you had to use USB devices, then it's best to slow them down to USB 2.0 and filter them out. If you were going to be using a USB DAC, then you'd be sharing the USB bus with other devices. This isn't the case for your setup as you are using optical. However, is it possible that cleaning up the USB bus power might make an impact on the optical circuit? Yes.
Since your input is optical, you don't have to worry about electrical noise. You instead have to worry about jitter. To reduce it, you'd want the sending side (iMac) to be as clean and stable as possible. Of course, this shouldn't matter if the DAC is designed to be immune to jitter. However, in the real world, things aren't as simple.
The only way to know is to try it yourself, preferably from a place with a good return policy. I similarly use Toslink from my TV into my DAC (Emotiva DC-1) in one of my rigs.