XMPP works for Google Chat in the sense that you can talk to other people who use Google Chat, but not Hangouts - at least not in my roster... So Hangouts is effectively a segregated quasi-XMPP thing that follows its own rules.

So far, there are no real alternatives to Hangouts if you want to talk to people using Hangouts, and that's it. And the XMPP third-party client "ecosystem" is pretty much fossilised...

As to ChatSecure and other alternatives, they all seem somewhat iffy. Before moving back to iOS, I wrote this: http://taoofmac.com/space/blog/2014/11/13/0830

...and it's still all mostly valid.