For example, "Race, religion, and gender, are still crucial factor's in someone's success"
Should there be a comma after "gender"? And is this a fixed rule?
In a word, no. Race, religion and gender form a compound subject and are followed by the predicate (in this case a complement) are still crucial factor's in someone's success. There should be no comma between a subject and its predicate.
This is a fixed rule to the extent that all descriptive grammars and style guides that I am aware of proscribe the usage. There's a good summary of the issue, with citations from various reference sources, on the Sentence first, with the exemplarily incorrect title:
Adding a comma between the subject and predicate, is inadvisable
Here is a brief extract:
Quirk and Greenbaum’s University Grammar of English, for example, says categorically that a comma 'cannot separate subject from predicate'.
And incidentally, factors is a simple plural, not a possessive. There should be no apostrophe.