Few questions regarding entities - Modifying a mod

[bulletbutter]

I have a few questions regarding vanilla entities.

1. Where do I find the health values of vanilla entities?
   
2. Are health values of entities affected by game stage?
   
3. Is there a value that determines which type of entities are spawn depending on game stage?
   
4. What is the max and min spawn rate for entities (I've modified some to as low as .001)?