Here is an email I got from Two Little Fishes on mixing media: "You can use any pump with a capacity of between 100 and 300 gph. This does NOT mean that you will pass 100 to 300 gph through the reactor!
You will use the ball valve that comes with the reactor to reduce the the flow rate to approximately 30 to 50 gph, on average. The instructions detail how to do that, and offer a slightly higher max recommended flow rate. The reason for recommending a pump with higher flow rate is to allow for head pressure losses that vary depending on the installation, and also to allow the user to periodically "flush" the media for a few seconds to loosen it up, which prevents channeling.
You can mix carbon and PhosBan, yes. We recommend using our HydroCarbon2, since it is one of the only carbons available that really does not leach any phosphate. Using other brands of carbon would defeat the purpose of PhosBan.
Place the carbon on the bottom of the reactor and the PhosBan on top of it. You can put a sponge (purchased separately as a set of replacement sponges) between the carbon and PhosBan layers. It is not necessary to separate them this way-- they can also be mixed. I normally mix them in a ratio of about 1 part carbon to 2 parts PhosBan."
Hope that helps anyone out. I know I will be doing it.