It would be good to know what clients you already tried. Jitsi? https://jitsi.org/
I haven’t tried myself as we don’t need audio/video, but i’ve heard it works for some people. And one contributor on this site is using jitsi librabry (or something like that) to produce his video conferencing solution Jitsi-videobridge. But currently it works in Chrome only i think.
As i said, we don’t use audio/video, so maybe my example is not very fitting. But we have been using Openfire + Exodus for 7 years (Windows XP/Vista/7) and now Openfire + Spark (~7 months, Windows Vista/7), around 170-200 online users. There were various issues with Spark, but nothing critical and many of them were fixed in the latest SVN updates. I’m deploying new Spark versions via GPO script during the PC shutdown (its installer supports /s silent switch), so administration is quite easy.