What I notice is that in general, if she doesn't eat much for one meal, she'll eat more the next or next two or three. Then every few meals or so, she'll clean out her bowl. So it seems to even out. She's not losing weight, so I think she's just self-regulating (which is better than Sara who'd eat out the entire kitchen if I gave her the chance!).
As for joint supplements, a lot of the vets here recommend Cosequin. If you can order online, one place to get it that seems to have pretty good prices is Entirely Pets: