Any non-jedi cant defeat any Jedi in a fight on the ground. Not even a squad of Mandalorians. Also, the order 66 seen was exaggerated. They would have sensed the emotions in the clones or what was about to happen.
Clones felt no emotion over the deaths of their Generals. Since they were just following orders, there was no emotion in the Force to warn the Jedi.
Also, there have been several instances where a Mandalorian has beaten a Jedi in combat. Jango Fett killed Coleman Trebor at the Battle of Geonosis, and also defeated several Jedi at the Battle of Galidraan, for instance. Then there were the Mandalorian Wars. Hundreds of Jedi fell to the Mandalorian warriors. Boba Fett supposedly defeated several Jedi in single combat and kept the lightsabers as trophies. The Jedi respected the Mandalorians for their fighting prowess, and rightfully so.