Lowest 11b is 1Mbps. Lowest OFDM defined for 11g is 6Mbps. You can increase that, usually to 12 or 24 since those are the basic rates 11g must support.
But you need to be careful. Raising rates doesn't actually decrease cell size since the preamble is always sent at minimum rate (6Mbps) regardless of the rate you set.
The least you could do is disable 11b. Do it now and thank me later! Then try raising minimum rate and see how far you can go with that. If you start running into problems throttle down.
I haven't tested with the new FW (9.7), but I think there is a small bug in it where if you set a rate of 12 or other and want to change back to 6 you won't be able to do it. It will notify you that 6 isn't supported, but you can set it to 5,5Mbps which is one of 11b rates which is just ridiculous.