From what I have seen the best solution for bridging chat protocols is a "tube" from https://sameroom.io - not free though.
I don't believe anyone's written a fully-functional XMPP connector yet though.
Any combination of Hangouts, Skype, GroupMe, Telegram, and Gitter is free to connect.
We also had a Freenode BridgeBot, but couldn't get around the max connections per user issue.
The overwhelming majority of customers connect to Slack though, so it wouldn’t matter price-wise.
I think I’m the only one reading #stripe from Skype :-)