I'm slightly baffled with your question. CCK is the modulation used in any case for 11M (either 11b or 11g AFAIK). The "sharing" in wireless is about Rx and Tx time on the AP. So if you have 11b clients in your 11g network, they will reduce your wireless network speed because 11g clients and APs will have to employ 11b compatibility (CTS/RTS...). And also, because of the slower speed, they will monopolize the AP's Rx/Tx (remember, this is a hub-like operation). You can disable the 11b protection, in which case the 11g radios will get the priority (or better said, 11g clients will not care about them), but then the 11b client throughput will suffer.
