Hi Hugo,
Neil is right, controller will send with the highest mandatory data rate.
If you have set 6M,9M, 11M, and ,24 as mandatory rates, the controller will send multicast packets at 24M only.when you set 9M to mandatory and the higher rates to supported, the controller will send multicast packets at 9M.
Check the need of your network/environment. To attain the multicast rate in the entire cell , It is good it keep it on 9M to be the mandatory rate, This will make the cell size a bit smaller. By doing this you can also prevent the network from B only clients.
Thanks,
Munish