In order to entirely disable remote repository caching, you need to check the ‘Do not Store Artifacts Locally’ check-box.
Unfortunately, it is not possible to disable caching of only a particular file.
The 4 parameters you are referring to are:
Keep unused Artifacts (Hours) - The number of hours to wait before an artifact is deemed "unused" and eligible for cleanup from the repository; A value of 0 means automatic cleanup of cached artifacts is disabled.
Retrieval Cache Period (Secs) - The number of seconds to cache artifact look-up results; This means that if Artifactory looked up an artifact from this remote repo and found it, it will not look for it remotely until the set amount of hours has elapsed and will fetch it from the cache (not relevant in your case where caching is disabled)
Assumed Offline Limit (Secs) - The number of seconds the repository stays in assumed offline state after a connection error. At the end of this time an online check is attempted in order to reset the offline status.
Missed Retrieval Cache Period (Secs) - The number of seconds to cache artifact retrieval misses (artifact not found); This means that when an artifact is not found on a remote repository, Artifactory will not try to retrieve it from the remote again until the set amount of hours has elapsed.
With regards to the ‘best practices’ on external repos, the default values set by Artifactory (added below) are optimized for most cases, of course this can vary depending on the scenario.