How can I be sure that Mac chat applications like Adium won't leak my Gmail, iCloud, AIM, or other account/password credentials?
As a rule, unless you've reviewed the full source code of an application yourself, you have to trust the developer(s) of the application that they're not going to misuse your login credentials.

Depending on your level of concern, you might set up a separate, dedicated Gmail account / login, exclusively for use with GTalk and related services. 

I'm uncertain how Adium's network protocol operates, but if it uses well-known servers and ports, you could also use LittleSnitch to inspect and approve outgoing network connections from Adium, to be sure that it is communicating only with places it's supposed to communicate with, i.e. official GTalk servers. (LittleSnitch inspection is only feasible with well-known servers, not with peer-to-peer based services.)