Is there a way to change the default instant messaging application, in Xubuntu, so that the status icons, in the task bar, apply to a different app, like Twinkle?
If not, can I remove the status icons, in the task bar, altogether?
The default IM app in Xubuntu (and all variants of Ubuntu) is now Empathy. The messaging-menu package is specifically designed to work with Telepathy, the backend for Empathy. Gwibber has been modified to specifically work with this interface. So, in short, there is no way to easily integrate unsupported applications into the menu.
If you don't want to see the messaging icon in the taskbar, open a terminal and run
I would strongly advise you take note of the action and be ready to reinstall it if you don't like what it did.