Hi Erik,
Many members of our team use Pidgin as their preferred XMPP client. As far as I am aware, this must be installed directly on the users' devices. I've never heard anything about installing Pidgin on the server itself and syncing it down to the users.
That said, please be aware that even through Pidgin, the chats sent through the SmarterMail domain can only be sent to other users and aliases on that domain. However, your users will have the option to add multiple accounts in Pidgin, so they could use their SmarterMail account to send chats to other domain users and use an alternative account to send to external domains. For example, on my personal laptop I use Pidgin, and I have my SmarterTools.com domain configured so I can chat with my coworkers. I also have my Gmail account configured so I can chat with my Gmail contacts.
Pidgin supports a ton of protocols, so you may be able to find a common one to use for chatting users who are not on your SmarterMail domain. For example, if you want to chat with other domain users who are on your same network, Bonjour might be a good option. It's a serverless messaging protocol developed by Apple Inc. based on XMPP that operates only on a local network. I've never actually used that on Windows, but from my brief review, it seems like it could work. Check out this Pidgin documentation for more info on what they support:
https://developer.pidgin.im/wiki/Protocol%20Specific%20Questions
I hope this helps!