DC, my short answer to your question is: no, I don't think Sonny was negligent. He had no reason to know that McCluskey was on Sol's payroll at that point, so he had no reason to believe that Mac would pull Corleone people out. And, they could have been held incommunicado for just long enough to let the murder happen.

In retrospect, Hagen was right to replace the gunsels with detectives licensed to carry firearms. But, I don't blame Sonny for not anticipating that his own men, carrying guns illegally, would have been arrested. His most urgent priority in the immediate aftermath of the shooting was to get some protection for Vito in the hospital.