The FG doesn't have a DOF preview lever, so it can't stop down the lens to working aperture for metering with the normal Nikkors with automatic diaphragm. And to mount a Non-Ai lens will be difficult, at least with some Nikkor lens as they have a ridge surrounding the mount that will clash with the AI tab on the body.
For lens without automatic diaphragm or preset aperture, then it can meter at the working aperture.
If you want a body that can use AI and Non-Ai lens, look for one with a movable AI tab: the original FM and FE, as well as the Nikon (Nikkormat series) EL2 or the FT3, as well as the F2, F3 or F4. They all came from factory prepared for that compatibility.
About the K3 screen: be aware that if used with those earlier cameras, some exposure compensation maybe needed as the meter system isn't made for that screen. The K3 is brighter than the previous K2 or K screens. See here: http://mir.com.my/rb/photography/har...ml/screens.htm