that might solve (nearly) everything.
Have you tried or considered Skype? It does all of what you say you're
trying to do. It runs on linux, Winders and Mac. It's free, though there
are paid options that allow you to dial outside the Skype network while
still retaining the free part for everybody connected to Skype. It does
conferencing, IM, file sharing (I've heard, not tried), connects to any
machine with a sound card and microphone, it supports some internet
phone equipment (but not SIP, I believe), it is portable in that you can
log on from anywhere in the world and use it, you can block or allow
specific users, you can search users, you can change available status
the way you can with IM, you can have multiple users running from the
same location (on different machines), you can have multiple identities
for each machine, etc.
I'm not trying to sell it, since it isn't for sale. It just seems like
the easiest way to go to get what say you want.