Kopete from KDE 4.3.1 does work alongside KDE 3.5.10 - I built a version that included the fix for Yahoo authentication and it ran fine, but I had to take care to use a completely separate directory tree such as /opt/kde4local that would not be used by KDE 3.5.
Unfortunately this didn't work with any emoticons which were essential for the person I was doing this for, so I ended up switching to Google Talk, which uses Jabber and worked fine with KDE 3.5.10's Kopete. Since Jabber is an open standard this sort of problem should not recur.