Neat! I'm actually working on a similar thing for one of my games; I'd love to know more!
To think about your question on 1 vs many companions, the solution I've found is in power. So basically you could have say a Tiger which can deal moderate damage, cause bleeding and cause fear. OR you could have a Bat, a Rat and a Cat which all cause minor damage but also cause bleeding, illness and superstition respectively. They're about the same power level, just spread out more which has its pros and cons.
Hope this helps!