I am firmly on both sides of this argument.

Sonny was negligent in that every plan should have a backup plan. While he was correct to put Corleone family people as guards, he should have had a Plan B in place. It took Tom all of a few minutes to have the armed guards and the potential of a show cause order in place after Michael placed the call to Sonny. Moreover, once it became clear that McCluskey was involved with Sol, Sonny still did not understand that Sol was probably invulnerable with a police captain as his security. It took Michael to solve this problem.

In Sonny's defense, it appears Vito had been in the hospital for several days before Michael's visit, and the use of Corleone guards seemed to be working. Don't forget these guys were remived when the hospital was on the graveyard shift, and more likely than not, based on what the nurse said, someone on someone else's payroll made a complaint that the guards were interfering with the operations of the facility. It would have been easy enough for McCluskey to keep the Corleone people from getting in touch with anyone. Remember there were no cell phones then!