I'm going to be the group stick in the mud and, suggest that you not install Mo'Creatures. I spent months tweaking it, leaving bug reports, working with friends on my server trying to get its catastrophically buggy Custom Mob Controls mod to play nice.
Some wonderful issues you can expect from installing Mo'Creatures:
1.) The Nether will be nearly impossible to explore. CMS wreaks havoc with Natura nether spawns quintupling the number of Nitro Creepers and Heatscar Spiders. It also ramps up the Thaumcraft bat spawnrate and NO, tweaking their spawn options does not work.
2.) Ocean mobs will randomly break for no apparent reason. When this happens you will have to manually program in every single biome type that you want water mobs to spawn in. After doing this you're going to run into problems such as, sharks spawning in 4 x 4 pools of water.
3.) Mo'Creatures does not support every Biomes O'Plenty biome. You will need to configure each mob's spawn group with the appropriate biomes those mobs should spawn in. There are over fifty biomes including mini-biomes.. It isn't worth it.
4.) Thaumcraft's magical forests will no longer spawn Pechs correctly. You can attempt to tweak this but, I've had minimal success. Either you wont find any or, you'll find so many that your performance will drop so substantially your game will probably crash.
5.) Many Mo'Creatures's hostile mobs are beyond buggy. Rats will clip into the ground, Golems will attempt to grab mod blocks that they're not allowed to grab (this results in a crash), Ogres will "smell" players at a far greater distance then they're configured to, etc.
6.) There are hostile ambient spawns. Not all ambient spawns will cease spawning at light levels greater then seven. Some of these ambient mobs will trample your crops or slaughter your farm animals. Many of them will simply bypass fences either by crawling through them or hopping over them. Yes, tiny birds will trample your crops..
7.) Mo'Creature's pets seek out their own death. At times I've had my kitties clip into the floor for no reason and suffocate. Litter boxes behave erratically. Half the time they'll simply up and vanish, clip into a wall or, slide out of the room you placed them in.
There's a metric ton of other cross mod incompatibilities, outright performance issues and, other strange bugs.
Until Dr. Zhark makes himself more approachable or fixes some of these issues I can't recommend anyone install Mo'Creatures in a large mod pack for any conceivable reason. The effort required simply to get the mod working just isn't worth it. There are far better mob mods out there, one of which posts on these forums (Lycanites Mobs).