I've successfully had Mo' Creatures, ExtrabiomesXL, and Biomes O' Plenty all work in Single Player together, however upon making a server with it, the client crashes with the following error:
https://dl.dropbox.com/u/1455347/Error/ForgeModLoader-client-0.log
https://dl.dropbox.com/u/1455347/Error/crash-2013-04-06_12.28.44-client.txt
Config files for the mods (if needed):
https://dl.dropbox.com/u/1455347/Error/BiomesOPlenty.cfg
https://dl.dropbox.com/u/1455347/Error/extrabiomes.cfg
https://dl.dropbox.com/u/1455347/Error/MoCProperties.cfg
This error does not happen in Single player and the error exists on the latest (1.5.1-7.7.1.639) Forge version.
I've tested it and this error doesn't happen when I remove ExtrabiomesXL and only have Mo' Creatures and Biomes O' Plenty.
If I remove Mo' Creatures, Biomes O' Plenty and ExtrabiomesXL work together fine too. It's just when they're both enabled the client crashes.