Verified Possible (Maintenance only). If an iPad Mini is currently running iOS 6, it can be restored to iOS 6 only if the user has saved their SHSH blobs. Otherwise, connecting it to iTunes will force an upgrade to the latest supported version (iOS 9.3.6).
If you are desperate for an iOS 6 experience, the most reliable solution is to purchase a second-hand original iPad (iPad 2 or iPad 3) or an original iPad mini that is already stuck on an older iOS version. downgrade ipad mini to ios 6 verified
Requirements that would be needed (rare) Verified Possible (Maintenance only)