I used to work at a large bank and they operated perfectly fine with Window's messenger. As does every other bank in the US, despite having tens if not hundred of thousands of employees. Some things require more specialized features like trade tickets (which Bloomberg does), but for chatting, you're not gonna be able to build a better solution than already exists. And if you can, then you should spin that off into its own business like Goldman did with Symphony [0]

The nice thing about being a client is that they support all this stuff and it gets better over time. Also something like Slack is absurdly cheap compared to the usefulness and productivity it provides.

[0] https://en.wikipedia.org/wiki/Symphony_Communication