Or just use Discord. Yes it is "meant" for gamer. But it does everything Slack already do. And better. My team use it.
Squeeze an IRC client into 400x400, and you'll still have plenty of room to read what's going on.
IRC sample (it's not 400x400, but you should get the idea):
http://www.ubuntugeek.com/wp-content/uploads/2010/07/smuxi-0...
Discord sample for comparison:
http://static.filehorse.com/screenshots-mac/messaging-and-ch...
I think there's a lot that could be done to make Discord more desktop real estate friendly. It's one semi-dealbreaker for me too. I can't have one single chat app require almost one whole new monitor.
Talking code in it just downright sucks.
It might have been a little while since I tried.
Discord isn't, right? It's a nicer Skype, if we're talking deployment and freedom?