If a tiger is an appropriate symbol of Jack in this chapter, what might be an appropriate symbol for Ralph? A lamb?

No, I wouldn't say a lamb. Ralph doesn't take things lying down. He does fight back. Ralph just isn't as manipulative as Jack. It's difficult to put an animal personality to Ralph. I think he certainly gets wiser as the novel progresses. Perhaps an Owl might work because, like Piggy, a quires more knowledge and like Simon he becomes quite wise by the end of the book.