USB DEVICES WITH THE 32U4?
Some important facts:


The 32U4 cannot act as a USB master, only a USB device (it doesn't have a USB OTG interface).
The USB shield is an SPI based device. It can communicate with any host MCU that can use SPI, as long as you can wire it in properly.
The Arduino doesn't have the processing power or memory to deal with the sheer onslaught of data a USB camera sends. 


So yes, you could use the USB shield with a 32U4 based board if you can work out the right wiring. However you will not be able to use things like cameras with it since you lack the processing power to do it.

For camera work you need something more like a Raspberry Pi.