Much to the point of the OP: if the new configuration is indeed 6F/balance Y, it's going to be much harder to get an F seat. The DC-9s have an atypically high ratio of F to Y seats.
From SeatGuru:
DC-9-30: 16F/84Y
DC-9-40: 16F/94Y
DC-9-50: 16F/109F
As for aircraft/aircrafts, this should be authoritative:
http://www.m-w.com/dictionary/aircraft