The possible other drugs would cause the problems especially SSRI's. But Klonopin would act as a counter agent to adverse SSRI reactions if they took enough Klonopin. I wish doctors weren't so eager to dispense SSRI's especially without educating patients and family about possible adverse reactions which can be like giving them LSD and in some cases if not detected cause death. IMO Benzo's are safer IF the doctor and patient understand how to use them properly.
I've taken a Benzos daily since 1994 and none of the things the Benzo fear mongers say has happened. Benzo's in general got a bad reputation because of Hollywood types using them and boozing it up. Not a smart idea. The key to successful Benzo usage is small but consistent levels in the bloodstream at all times. IOW high dosages defeat the purpose.
I've seen a severe Serotonin reaction before and I am also among the category who can not take them due to sensory processing damage. I find it sad that high schoolers know how to induce a Serotonin migration using such things as OTC cold meds yet most ER doctors do not know about Serotonin Syndrome in persons who use SSRI's legitimately and have adverse reactions to them. All to often the attendings mistake an adverse SSRI reaction as acccute pyschosis and in doing so start a medication treatment protocol mking matters far more worse.
The person I know who had the Serotonin Syndrome had PTSD. The doctor kept upping the SSRI and then added another. The fact she was also taking Xanax likely saved her life.
The most important thing with PTSD is finding a good therapist to help the person work through it. That does far more good in the long run than anything else.