Wingtip Missile Rails

A couple questions about wingtip missile rails have been on my mind of late and maybe someone can help me with the answers.

I look at the aircraft that have them (F-1C, F-5, F-16, F-18, Rafale, Gripen, etc.) and in their respective weight and generation classes they’re all as agile or more so than their contemporaries who don’t have wingtip rails. Is there a reason against using them? I looked at the Jaguar and from the dimensions it appears that it could be modified to use wingtip rails and allow the ECM and Chaff/Flare pods to be relocated to the overwing location and thus freeing up the outer wing hardpoints for ordnance or a self designation pod.

The other thing is similar to the wingtip rails and that’s winglets. Would they help or hinder a combat aircraft? I saw a graphic of a MiG-31 that had what looked like huge winglets as part of an EW (IIRC it was an EW fit) package.

Finally…if winglets are viable on combat aircraft, could they be combined with wingtip rails (the rails would be outboard of the winglets)?