Update on Gimbly - the soft poop has resolved yet again. If it occurs again, then he'll have to see the vet. Can guinea pigs have various complicated GI issues like cats and dogs? One of Gimbly's triggers seems to be veggies. All the other pigs are fine with the amount I feed, but sometimes it triggers the soft poop. As a result I feed Little Bear and Gimbly slightly less veggies than Chico and Amigo.
His poop doesn't smell, but its very big, clumpy, and wet/sticky. Sometimes there's formed pellets, sometimes it's mushy pellets, other times it's just a mushy pile. It isn't liquidy. A few times it appears like a dark green, but that's not often. As he got better, there was more normal pellets and less mushy. I know it's definitely him because he's twice the size of Little Bear and has big pellets compared to LBear.