squirrel is better than chicken as random shield.
You have to watch out for things like frag grenade. You must micro and watch your animals very close, I find most of my animals injure are actually from friendly fire after they gang up on the enemies. Buy huskies and retrievers, they are easy to train and breed rather fast. You will want multiple colonist with high animal skill and someone dedicated to cleaning, having a bunch of dogs running around will get your base really dirty really fast.
Snipers work really well with animal army. They can pick out things that are too dangerous for your animal army to handle.