I have a custom set of smilies from AdiumExtras (GigaSmiley). When I put in one of the GigaSmilies, the person I am chatting with does not see the smilley only the characters for it. My contact is on windows.
Smilies are not sent over the network. All you send are the text, while Adium replaces some text on your computer to the relevant smilies. Hope it helps