KDF is a difficult media to do properly, and unfortunately very few companies do it properly. Personally, I think it is an amazing media that is greatly underutilized, but since so few people know how to use it, the media is highly problematic. Check out this link, it is too my system which has a KDF tank done semi-properly. http://www.terrylove.com/forums/show...ight=prototype
Kdf requires a heavy backwash. A 10" tank would require a backwash rate of 16 GPM. My own system uses a 7" diameter tank that only requires 8 GPM to properly backwash. Mixing KDF with other medias should not be done, but a lot of companies claim to have "the right way" figured out. You are on the right path. Seperate tanks for the different medias. http://www.kdfft.com/images/kdf_POESheet.pdf This is a link to the KDF application guide with proper backwash rates. In my system, I have been getting 10 years out of the KDF media with backwashing it once a month. KDF recommends a much more frequent backwashing. My design is obviously not normal, and it is designed to push the limits of what can be done and to test the different adapter types. For your application, assuming your water flow is normal (no mega showers, etc.), an 8x44 tank with 57#'s of KDF, a garnet underbed, and a good backwashing valve (7000 or 2510 is ideal), you should do ok.
Hope this helps.