Protheans merely proceeded the current cycle, but they were still part of the cycle. I don't know why you'd compare them to Forerunners. Leviathans are probably the most powerful beings that have had screen time in the ME universe. Peak predators, they can destroy massive dreadnoughts and enslave entire civilizations with their minds. They were only forced into the dark corners of space due to their arrogance, believing that their creations, the REAPERS, wouldn't or couldn't turn on them. Now that sounds a lot more like the Forerunners.