The Cokin 67-72mm Step Up Ring will work well to attach the Canon 72mm 500D Close up Lens to a lens with 67mm filter threads: http://bhpho.to/1r3Evyz

Canon does not manufacture a 67mm close-up lens. If you are looking to purchase a close up lens for use on your Canon EF 70-300mm f/4-5.6L IS USM Lens, I would recommend purchasing both the Canon 72mm 500D Close-up Lens, B&H # CACUL500D72, and the Sensei PRO 67-72mm Aluminum Step-Up Ring, B&H # SESURPA6772, for your usage needs. The Sensei PRO 67-72mm Aluminum Step-Up Ring would be the adapter that will allow you to connect the Canon 72mm 500D Close-up Lens onto your Canon EF 70-300mm f/4-5.6L IS USM Lens.

To use this close up lens on the Tamron 18-270mm lens, you will need to use a 62-72mm Step-Up Ring, such as the following made by Sensei: http://bhpho.to/1ysvihv

The 70-300mm Sigma has a 62mm filter thread, so none of the 3 will work. Here is the link of the compatible close-up lenses:http://bhpho.to/1cWGyxz

There is no lens cap included with the Canon 72mm 500D Close-Up Lens. You can use any lens cap for a 72mm thread, such as the following product from Sensei: http://bhpho.to/1pYJsDf

The 250D & 500D have long been the best option for close up filters but these do have optical properties and are best used as recommended. I see no reason why you could not use it on the 18-400mm lens at the longer focal length range. Used at the wider end, you may see a reduction in image quality at the edges of your image.

Yes, this Close-up filter is able to fit on your Nikon 18-200mm lens and is most useful between the 70-200mm range on this lens.

We would not be able to speak to this specific photographers usage.