This is a great idea, and the half-breeds need to have some strengths and weaknesses. These should be in two parts, the animal part and the mortal heritage. For example, a human - spiderling half-breed has a higher skill bonus from the human heritage, while an elf - spiderling will have the elven speed bonus, as well as a slowing weakness and night vision from the spider heritage. Also, only certain races can have access to certain half-breeds (i.e. dwarve can only have satyr, spiderling, and minotaur half-breeds, while elves can have satyr, spiderling, and centaur half-breeds)
Other wise, this would be a great addition to the game.