Sadly no. The newer the iPhone is the bigger it becomes.
No. Apple don't make anything to be compatible between phones....its called corporate greed
Too bad you can't seem to stumble across the Apple website, or you could look up the dimensions for yourself.
It's HIGHLY likely that it will fit. Many iPhone 8 cases will note compatibility with the iPhone 7 & 6/s, as the size difference between them are around 0.01 inches (or ~0.2 mm) between the three phones (as noted by Apple themselves when you use their Compare Tool at [ Apple.com Link ] )... placing the phones within manufacture tolerance ranges.
The iPhone 8 case may not provide a perfectly snug fit for an iPhone 6 (it may feel ever-so-slightly loose), but it's close enough that you most likely won't notice it. The only potential issue may be with the rear camera, as the placement on the iPhone 6 is a different location compared to the 7 & 8. If the camera cut-out is circular, it won't fit... If it's more rectangular, it'll be compatible.
No, it most likely will not fit.
No. The 6 is a different size.
If it was an iPhone 7 there would be a chance as the 8 is just 2mm bigger all round.
No the dimensions and placement of components is different!