Layering seems harmless. Maxis has used for some breeds about a dozen layers. The impact on character file size is minimal, since it's not actually storing every single layer's texture: You can only choose from preset textures anyway, so the game merely needs to store what you chose from the selections.
However, it *WILL* cause a processing hit to use a lot, so I wouldn't fill a lot with 30 15-layer pets.
Well, that's good news about no long term problems, but not so good news about the processing.
I did a quick check on the cats for layers and the Himilayan cat breed that came with the game is 24 layers. That's probably tops. But any cat with boots, additional leg color, and facial markings is going to be up there in layers, especially if it has pink ears.
The dogs are not slouches, either. The Australian Cattle Dog is 21 layers. The Rottie is 12, and the Collie 16, the Aussie is 18. These are all EA Games default breeds.
So, I don't feel so bad. Here are my layer stats:
Dogs:
AMjoie Poodles change by color, but range from 1 (blue) to 5 (blush apricot)
AMjoie Chocolate Lady Julie Spaniel 9
AMjoie Lilac Lady Julie Spaniel 10
AMjoie Liver Rolf Setter, both miniature and standard 8 (will be uploaded soon)
AMjoie Black Jacque Terrier 2 (will be uploaded soon)
Cats:
AMjoie Tonkinese 5
AMjoie Aby 15
AMjoie Silver Snowshoe 15
AMjoie Lilac Snowshoe 19
Since my highest, the Lilac Snowshoe, has as many markings as the EA Games default Himilayan but is 5 fewer layers, I don't feel bad at all. I've been frugal. LOL