Your best bet is to use OTR encryption over XMPP or another messaging protocol of your choosing. Also configure clients to use SSL/TLS.

You can learn more about OTR here: https://en.wikipedia.org/wiki/Off-the-Record_Messaging

You will actually be using a different client on your iOS devices compared to OSX. Just use the same account/protocol and you will be fine. On MacOS you will probably want to use Adium. On iOS use ChatSecure.