With a 50mm lens 8-10sec is really the max you can use before trailing gets really noticeable. At 55mm I'd try at 8 seconds and starting with an ISO of 800 is a good start, you can try higher or lower ISO's and see which looks better in the end. If you still see trails try 6 seconds.
Give those settings a whirl and see what happens, 55mm for 8 seconds on ISO 800.
Hope that helps a little bit.
