The big thing that Alamy look for are dust and rubbish spots on your image. They seem to be quite tolerant of noise and grain. I've submitted scans of transparencies to them that have been rejected by both IS and SS, and they've accepted.

In fact I've never got a tranny scan past the SS inspectors, which is a pity because I've got thousands of useable trannies (scanned with a Nikon Coolscan 5000, so not rubbish scans). IS have accepted a few. I even got an inspector's 5/5 for one once