I was right about the stretchiness of the garter stitch. One thing that I did notice is that size of stockinette stitches versus that of garter stitch caused two things to happen to the green fish. First is that the shorter height of the garter stitch caused the green guy to have a really nice shape: he's short and round like a Goldfish cracker. Second, the change to stockinette stitch for the gills produces a huge bubble in the fish. (It is why the green fish looks so skinny in the picture with the blue fish.) I was able to get the fish flat for a few seconds, but any handling or even a little time and the bubble returned. I'm not sure if stitching the fish together would do much for the afghan, but it doesn't look like it would lay flat. (I tend towards acrylics. A yarn that blocked better looks like it would work - leaving the fish under a cookbook for a while has helped quite a bit.) However, the bubble won't be a problem in making a stuffed fish.
The reverse stockinette fish doesn't quite have the "goldfishiness" of the green guy, but still has a nice fish shape. It is curling in the picture, but that is simply due to it being stockinette. I think I'll have to change the pattern of increases and round out the head and body a bit, but overall I do think this pattern is a good starting point. I will keep the stockinette for creating my own variation of a stuffed fish - the curling doesn't matter once things are seamed. I'm not sure whether I like the reverse stockinette (with stockinette gills) or stockinette (with reverse stockinette gills) better. The reverse stockinette looks closer to the original, but the gills strike me as being sideways, but don't really look like a fin. Here's a picture of the fish with the reverse stockinette side up: