The easiest thing is to merge contacts, so all the contact methods for one person appear under their name, along with green circles when they are online via that service. Then when you click the name you can chose to call via VOIP, Mobile (GSM/3G) or skype or whatever.

But if you actually do want to keep them separate you could install pidgin (a separate chat program) so they never show up in the contact list. Then if you go the built in IM client via the status bar you can disable those accounts and the IM contacts disappear from your contact list.

The downside to this method is the built in client is actually very good at staying idle, using minimal CPU time, and using the notification light etc. Pidgin isn't quite as schmick.

The other alternative is to use the "Groups" the other poster suggested. Probably the best way: Click "All contacts (###)" on the title bar -> Groups -> choose the service.

Try KOffice.

There is a way to set contact specific ring tones. I don't know what the app is called.