"Can you put animated tiles on layers 6 and 7? I'm pretty sure you can't on layer 8, so that's why I was wondering."
You can have animated tiles in any layer at all, unless it is textured. The only way you can get anything close to an animated textured background is as described in this horribly written article, near the end, where you can have a textured background in Layer 7 and put animated tiles in Layer 8, and the textured background will only appear where there are non-transparent pixels in layer 8... okay, more detail than you wanted, I'm sure. Yes, you can have animated tiles on layers 6 and 7.'

"But I'm wondering: Do the animations take one tile or one row? "
Ha, I'd never thought of that, but it's a very logical question. They take up one tile each. Beyond the 1023 total tile limit, I think you can only have 255 animated tiles period, but that's almost never going to be an issue.