Yes you can the XGATE with 29-bit CAN identifier, otherwise it wouldn't be CAN.
The only difference the mcu makes between 11 and 29 bit is that if you are using the message masks and filters, you need to set them for either 11 or 29 bit. If you don't use either, you don't need to worry about it at all.
What do you mean with "embarrassed documentation"? It is written by engineers for engineers, so it won't exactly be masterpiece fiction. It contains all you need to know though: if you are an engineer who knows CAN, you shouldn't have any trouble understanding it.
