It would be a huge advance in usability if Whonix had a pre-installed IM client. Gajim, though not perfect, is by far the best choice.
Notes on our wiki https://www.whonix.org/wiki/Dev/Gajim
It seems that stream isolation is the only blocker to including it. We can discuss that here or on the forums (https://forums.whonix.org/t/should-strict-stream-isolation-by-a-requirement-in-whonixs-default-appliation-policy/3940), but I'm thinking that "stream isolation as a requirement" keeps our hands clean in the event of a highly sophisticated attack, while letting our users fumble in in the dark (such as installing pidgin, or installing gajim but not configuring it) or giving up.