Edit - Seems like the mod doesn't read the config.
Edit 2 - I finally found out how to change the spawn rates.
I highly recommend setting the bunny spawn to 0 to prevent unplayable lag due to rabbits breeding like rabbits do.Here are the 2 files you need :
Put them both in the ItemCraft\ICSripts folder on your server, replacing the file thats there.
The plugins.ic file is set to run ONLY the Mo Creatures mod - if you are running any of the others, you will have to edit the file & remove the '#' from the beginning of the line containing the other mods you wish to run. Plugins.ic also contains the changes needed to run the Trees mod - if you are running it that is, if not, it doesn't matter, this part will not interfere with Mo Creatures.
The oldscript.ic has already been edited to the correct values, but you are free to re-edit if you wish.