Are Minox negs 9mm? I used to shoot a bit of Minolta 16 film (which is 16mm motion picture camera film), so maybe a bit bigger than Minox. I use a Nikon Coolscan 9000 and originally would lay the negatives down on a Newton glass carrier (for 120 film) I had for the scanner. It worked pretty well, but there was a lot of post processing to extract the little images from the large scans. Turns out Nikon made a 16mm film holder for the Coolscan 9000, and I was able to find one of them (they're pretty rare), and now I use that.
If you have a flat glass negative carrier for the Minolta 5400, you might try laying the negatives out on that. And maybe Minolta made carriers for the tiny negatives as well.
Good luck. It's surprising the quality that can be extracted from those tiny negatives.
Best,
-Tim