I want to remove the Evolution group and replace it with ThunderBird, remove Chat and replace it with Pidgin, and finally remove the Broadcast group.
How do I do that?
There is a fantastic article here on how to remove unwanted entries from the menu. I'll give you a short summary of it, but it's worth a full read.
You'll want to run these in Terminal:
This will remove CHAT, BROADCAST, and EVOLUTION from the menu. I can't tell you how to install the Thunderbird one, however; this command MAY do it, but there's no promises. I don't use Thunderbird, so I can't give an absolute answer.