Is it possible to assign a Mail signature to specific recipients? I want my signature to appear in emails I send to some people but not appear in emails I send to other people. Thanks.
No. This isn't possible with a standard install of OSX Mail.app.
What you can do, as I am sure you are already well aware of, is set up multiple signatures, and manually select the signature that you desire to use. What you can do in the case of wanting no signature is set up a "signature" labeled "Empty", and then put nothing in there. Then have that set to default, or the other way around.