You can type to filter in the Contacts application, but this issue could use some looking into.

As far as dealing with having 300 screen names in your contacts list I'm not sure, I've been too lazy to deal with it, lol. Most of the people I talk to on AIM I also have contact cards for, so I just merged the info. Creating real contact cards would be one option. I'm not sure if theres a way you can make it not add the screen names to Contacts. I will mess with it later but I'm guessing once they've been added removing your AIM account won't remove all the contacts it created either.