Did the manderlys make the pact with the starks before leaving the reach ?

No, it’s pretty clear it was after:

“A thousand years before the Conquest, a promise was made, and oaths were sworn in the Wolf’s Den before the old gods and the new. When we were sore beset and friendless, hounded from our homes and in peril of our lives, the wolves took us in and nourished us and protected us against our enemies.”