"are they interchangeable? i have heard that they have different input spline counts. anybody know?"
yes and yes
The problem with an answer to your question is, well, that it really doesn't answer your question. That's because we have no idea what you need to put it in and what it came out of ...
No. the input splines are the same but the length of the input is different. The autos have a little longer input than the manuals. Thats why Tera offers a seal "extension" for its teralow kit for autos. Some guys are looking for the auto planetary inputs so they can add clocking rings and still have enough spline engagement. I'm not sure its ALL the autos though.
So the answer is it MIGHT bottom out. Measure the length of the input.