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Is it possible to crop thumbnail set width and hight auto?

Not without some custom development. You'd pretty much need to modify the entire attachments system (and all of the layouts). Everything is built around having all thumbs being the same dimensions.
I almost found a solution for my problem. I just deleted the "crop method" in library/NFLJ/Showcase/Model/Attachment.php.

Now only the "fit method" works. (Fits image into width and height while keeping original aspect ratio. Expect your image not to use the full area.)
I hope I will not have errors in the future.
P.S. "Rebuild Showcase Thumbnails" still uses the crop method