All creates that have lesser HD that 1 is considered 1.

Normally Monster Manual (or Dungeon masters Guide) provides such information, but i a not sure which one... Creatures get good or bad scores according to their types...

[This document][1] provides what you need, also it states HD categories as 1 or *less*, 2, 3 etc...  


  [1]: http://www.d20resources.com/modern.d20.srd/creature.types/