There are dependency issues as the harmattan metapackage among others depends on them. I said "suppose" because I haven't tried removing those dependencies by hand from /var/lib/dpkg/status. After doing that it would be possible to remove only the twitter and facebookqml packages using dpkg and leave others be.

It is possible that everything would work and FB+twitter simply disappear from menus and apps that use them, but it could also be that integration isn't only at the package level and that removing facebookqml and twitter packages completely breaks some functionality on the phone. (webupload, contacts). So try at your own risk and be ready to reflash.

Yes, there's support for XMPP OOTB as Harmattan uses empathy as it's IM backend. To add your account you simply use the commands provided in my previous post.