First off, thank you for the advice concerning spriting/lighting/etc earlier! I've since made some headway on the breed I'm working on (despite a few setbacks), but I've gotten stuck on a worry of mine.
I know from experience that breeding creatures with different sprites will sometimes get you weird, disjointed/dismembered creatures if they're using certain breed sprites in combination with other breed sprites.
While I'm not worried about ATTs becoming a problem with pure-bred creatures, I'm worried that my breed would become prone to this problem when crossbred, and I want to try and prevent the problem before I've made too much progress.
What do you know about how ATTs interact with other ATTs from other breed slots? Can a problem like this be easily fixed?
Each Norn body part has an ATT which tells it where its joints are. So a Norn head has a "neck joint", and a Norn body also has a "neck joint", and the game positions the head on the body by lining them up with one another. Easy, right? I'm sure you probably know that much already. The reason some breed combinations "go wrong" is that in the past, not all creators were consistent about making sure the joints were in the same relative place on each body part, compared to other breeds.
A Norn with (for example) a neck joint located 10 pixels too far down can still look visibly normal in the game, when both its head and body ATTs match up with each other. But when you get a crossbreed with that breed's head and another breed's body, bad things will happen. Its head ATT specifies that the neck joint is 10 pixels further down than usual, but its body ATT specifies that the neck joint right where it's supposed to be. As a result, the Norn winds up with a floating head.
If your breed has the same proportions as an official breed and are positioned the same way on their sprites, you can just copy an official breed's ATTs and avoid the whole issue. If not, that's okay too! As long as you can get the hang of how ATTs work in general, I don't think making your breed compatible with others will be too difficult. But I do strongly recommend using a graphics program like Photoshop, Gimp, or even Paint to keep track of where connection points are located on the sprites--not just for your breed, but for one of the official breeds too, so that you can get an idea of where joints are supposed to go.
And finally, you can always use the Genetics Kit to test hybrids in-game. I had a lot of concerns about how Gaius crossbreeds would look, for example, because he's built so weird. Because the Genetics Kit allows you to specify species/breed slot for each body part, and then create an egg in-game, it's a breeze to gengineer a test subject with any combination of body parts you want.
I hope this answer has been helpful; ATTs aren't always easy to explain without visuals, but hopefully they don't seem too scary.