You shouldn't need to add rice once or twice to harden its shit it should be solid anyhow. Important thing is to find a food your happy with and stick with that one brand. Gain 20 stick to gain 20, if you keep chopping and changing its grub it will end up with the shites. Dogs get used to there grub. Yes rabbit will be fine to supplement gain 20 with, I even put in some fish, chicken, beef mince and mashed up veg when we have a roast.