I think that you (or your source) are reading too much into the actions of Quakers in Pennsylvania. Quakers like all people are fallible, and Quakerism especially embraces the idea of continuing revelation and moving beyond previous barbarisms. There were early Quakers who did own slaves for example. While there were actions that us modern Quakers would not condone, I don't know of any interpretations of history (other than perhaps the book you cite, which I admittedly have not read) that indicate that the Quakers of Pennsylvania engaged in some kind of barbaric war with the Scotch-Irish. While they still wielded considerable power they were never the only residents of the colony and it by-and-large still followed the norms of the day. Quakers were proponents of (what were thought to be at the time) humane punishments such as solitary confinement. I'm not going to condemn the idea of pacifism due to the fact that we were unable to obtain complete purity of fact two hundred years ago in an entirely different society.