You can't defame former employees but you can say true things about them. State laws, generally speaking, do not prohibit you from saying (true) stuff about former employees, in fact some have clauses protecting you from defamation lawsuits. At the most basic level it's a 1st amendment right, the Constitution doesn't have a secret section on employment law.
Even if what you are saying is true, that still doesn't mean it can't lead to a lawsuit.