Are users required to use the built in wallet? 

 Yes. Keychat automatically stamps messages, so it requires the wallet to have an ecash sat balance. 

 But I can use my own mints, right? 

 Of course. You can add as many cashu mints as you like. 

 is there a directory of mints somewhere? 

 https://bitcoinmints.com/ 

 dont know if this cam really be considered a social client...more a messaging client imo 🤔