Thanks, all. Your help through all this has been invaluable. You've given me theories to test and avenues to explore, plus support without recriminations over my admitted ignorance of feline digestion. I've had ratties for so long that I'd gotten too used to omnivores with cast-iron stomachs. Rats can and will eat anything.
As things stand, Artie is still gassy, but it isn't near as stinky as it was before. He's having no more loose stools; what he's producing now is firm and normal-looking. He was a little off his food for a couple of days, which I suspect might have been an effect from getting his booster shots (both animals and people can feel a little off after getting a vaccine... I always get sick after a flu shot), but now he's back on track. His energy level is even higher than it was, and it takes hours of concerted high-energy play to tire him out enough so that he isn't bouncing off my belly when I'm trying to sleep.
We're examining the possible gluten/carbohydrate intolerance now. If this doesn't clear up his troubles in a month, we're going to start testing him for a protein allergy. If that doesn't do it, then we'll see if he responds to the normal treatment protocol for Inflammatory/Irritated Bowel. If that offers no answers, the vet wants to do an ultrasound to see if there's anything anatomically wrong. She suspects that it is most likely an allergy to something, but it will take time to figure out just what.
Alistair the rat is likely looking at surgery to have both upper and lower incisors removed due to his being unable to gnaw them down himself because they're so badly aligned and poorly formed. They're not normal teeth by any stretching of the word. Two grow abnormally fast, thick and tusk-like, and they push the other teeth out of line. His little mouth hurts a lot, and he gets meds for pain. Every 2 weeks he has to be knocked out under anaesthesia to get them trimmed with a Dremel tool. Repeated anaesthetics are hard on him, so this is why I'm trying to get info on surgical dental extraction for him.
The odd thing about Artie and Alistair is that they seem to be friends. Close friends. Artie doesn't react at all to the other rats, but he grooms, plays with, and sits nose-to-nose with Alistair. He's never rough and his play isn't predatory... he lets Alistair chase *him*! It really is charming, though I don't let this happen unsupervised.