There are no "rules" in model railroading, and everybody is free to do their own thing. But, without a temporarily empty yard track (normally called the Arrival / Departure track), you will have to make up trains on the mainline, and that can be a real pain in the a**.
I know you mentioned using FasTrack and they don't offer a double-slip switch, and I know that many people on this forum use FasTrack, but I highly recommend looking at the numerous threads about which track is best. The overwhelming majority recommend a combination of GarGraves flex track and Ross switches, with a close second being a combination of Atlas track and Ross switches, for a variety of reasons. I think Atlas track usually sits in second place because it is not always readily available and its switches just aren't reliable enough.
Glad you like the idea of a lift-up turntable for access to the back.
Chuck
