Alex - All GSuite services enabled for your domain are accessible when authenticating through Okta. But as of now, we have no plans to include a separate Hangouts app.
if this is something you'd like to see, I encourage you to file a feature request in the Okta Community. Once a feature request is submitted it is visible to other Okta admins, who can vote on it. Using this method will allow you to monitor your request throughout the process.