They were shot using a tripod at F11 with an exposure of 15-20 seconds and
auto focus (should use manual I guess). Would a smaller aperture be better or is that overkill and do you lose something if the exposures are too long?
When people talk about shooting on manual, they usually still use auto focus. Nothing wrong with using it. While setting up manual focus in a shot like this wouldn't be too difficult, auto is a life saver when moving people come into play.
Manual or autofocus would be fine. While it doesn't matter too much here, consider using manual exposure when shooting on a tripod with a remote shutter release.
When you handhold your camera, you normally cover the viewfinder with your eye. If you put the camera on a tripod and shoot with a remote shutter release, the viewfinder is exposed. It is possible for light to come in through the viewfinder, bounce around inside your camera, and throw off your exposure. The light will get blocked once the mirror flips up (which happens when the picture is being taken), so it won't be in the picture, but it can cause you to underexpose.
There several ways of dealing with the problem. One is to set the exposure manually while you are looking through the viewfinder setting up the shot. Another is to cover your viewfinder before you press the shutter. Most cameras either have a viewfinder cover built into the camera strap or they have a built in viewfinder cover.