A: FireWire 800 is backward compatible with FireWire 400. All you’ll need is an adapter — usually a 9-pin to 6-pin — or a cable with the two different plugs. We carry several different cables here at Sweetwater for this purpose: Belkin and Hosa make two examples that work well.
There is no “danger” to connecting a FireWire 400 device to a FireWire 800 device.
However, there is a potential downside: When you connect a FireWire 400 device to a FireWire 800 device, the FW800 “downshifts” to operate at FW400 speeds. This isn’t a big deal if you’re, for example, connecting a single FW400 hard drive to a FW800 computer. But, if you have a FW800 computer with a FW800 hard drive connected, and you hook up a FW400 audio interface, the whole thing will slow down to FW400 speeds. Whether this is a problem or not depends on your situation.