SONY NEX-6 16mm-50
Basically, Can one use the fish-eye/Ultra wide teleconverter with the new 16mm-50mm kit lens of NEX-6?
No. The VCLECF1 is designed for the SEL-16F28 pancake lens only. It does NOT mate with the 49 mm threads - so an adapter ring will not work either.
I see three alternatives. Buy both the SEL-16F28 and VCLECF1. This works fine and you get a fair to good 16 mm kit lens too. Price from $260 to $360 depending upon the SEL-16F28 price.
Or get a Rokinon FE8M-NEX fisheye lens for about $300.
Or get a Rokinon FE8M-C (Canon version & manual control) for about $230 and add an adapter: RainbowImaging Canon EOS to Sony E-Mount - or equivalent. This is what I have. It's a great value if you already have a Canon DSLR. There are Nikon versions too.
Rokinon is made by Samyang. ps: You can get good (not great) 380 degree panoramas using the Rokinon lenses with the NEX sweep-panorama mode. It takes practice! - John C.