I installed Kontalk legacy and it just works like 3.x beta version: It doesn't show 2.x contacts and if I invite them it sends a message directing them to Google Play, which has 2.x version.

Maybe I misunderstood Kontalk's XMPP capabilities: I thought it could work as XMPP client and I would be able to talk with other XMPP contacts outside the Kontalk network.