Is a place of employment allowed to tell employees that they are not allowed to talk to ex employees?

It's probably not illegal for the employer to have such a rule.  Whether is enforceable on a practical level is another thing altogether.  How would the employer ever find out?  If the rule were violated, could you be fired?  Yes.  I doubt the employer would violate any type of law for firing you for that reason.  I do think you'd win your unemployment compensation claim if the employer challenged it though.  I doubt a violation of this rule would constitute "willful misconduct" such that you would be ineligible for unemployment compensation.