Although the confederates were outnumbered they continued to win major battles
Yes, I would say that good leadership was key to Imperial Rome's success because to be able to conquer such a vast area north of the Mediterranean stretching from Spain (and England) in the west to Palestine in the east and to be able to exploit the mineral deposits of many of these countries and to control the areas conquered without major rebellions must have required good leadership.
55,000 were killed, captured or wounded.
I did research and it didn't give me a number for just killed.