I took the cats to the vet on Saturday to check out the blood in their stool.
The vet took a fecal exam from both of them, and it came out negative. Since I only saw the blood once, she said it could have been just a broken blood vessel and nothing to worry about, unless they seem to be behaving differently. I should keep a close eye on them to make sure it isn't something that happens frequently.
I also asked her about Lewis' weight and annoying crying. He is 11 pounds now, and according to her still at an okay weight. She said she doesn't want to see him gain any more weight, but since I'm already being careful not to overfeed him, then I shouldn't worry about getting him down to under 10 pounds. (Side note: The difference between my last vet and this one. Cats with weights over 10 pounds were flagged as "obese" at my last vet. She constantly reprimanded me for his weight, which frustrated me because I've always been careful not to feed him too much. I appreciated that this vet was able to give me an honest answer based on Lewis himself, and not on just numbers.) She told me not to worry about the exercise bikes yet, but to supplement his food with a weight-loss food that would help fill him up without giving him the calories. I'm supposed to do about half of his normal food and half of the weight-loss food for now, so that's been an interesting juggle. :-)
As for the thyroid problem that I wondered if Lewis had, she said it's very rare that a cat as young as him (3 years) would have trouble with that. She wouldn't discount it completely, but I should watch him and if I ever notice the signs then I should bring him in for a blood test. More things to watch. :-) I would like to write a blog post about the signs of thyroid problems for cats, but that will have to come later!
Overall, Lewis and Ebony are quite healthy cats and the vet said I take good care of them. That's happy to know, and I hope that means they are with me for a long while yet. :-)