Is the area with all the xenomorphs in the missing derelict in the same place as on an Engineer ship? I thought they might be the same because it has the big gun-like star map found in Prometheus.
Yes, the derelict ship on the surface of LV-426 (in Aliens) is indeed an Engineer ship. In the film "Prometheus" and its accompanying literature, we learn that these types of ship were part of a entire class of vessels known as "Juggernauts" which the Engineer race used to explore the galaxy.
Several near-identical vessels are seen in Prometheus, along with the one that the titular ship rams.