There are some really good ones at Poliigon, I can wholeheartedly recommend those.
If you have found a bug, please notify me! There is a button on the store page to send me a message. It is really useful to include the error message you received (if any) and a way I can reproduce the error myself.
Yes! Put them in the same folder, and make sure they have the exact same name except for the last two characters. Currently, SMUDGR supports 1K, 2K, and 4K resolutions. So a filename could be, for example, MyTexture_4K.png. SMUDGR will display a message if you try to load a resolution that is not available in the folder.
One more important thing: The thumbnails shown in the interface need to be added too. Name these the same as stated above, but with 0K at the end instead of the other resolutions.