If a colleague uses classic Hangouts, you can directly message them from Hangouts Chat. You get the direct message in Hangouts Chat, and your colleague gets the message in classic Hangouts. This compatibility applies only to direct messages. Group messages can't be sent between Hangouts Chat and classic Hangouts.
If you also use classic Hangouts
If you use both classic Hangouts and Hangouts Chat, you get message notifications only in the service you send a message from. For example, if you send a message in Hangouts Chat, reply notifications for that message show only in Hangouts Chat. If you send a message in classic Hangouts, replies show only in classic Hangouts.
Note: To message people outside your organization, you need to use classic Hangouts. The ability to chat with external users is coming soon to Hangouts Chat.
Editing and deleting messages
If you edit or delete a message in Hangouts Chat, the message isn't edited or deleted for the user in classic Hangouts.
If you chat with a classic Hangouts user who changes their chat history setting, a new conversation thread starts in Hangouts Chat to reflect this setting change.