Reading through the steps you are taking, I can see too much that is wrong, but the young girl in the top left corner appears to have been upsized and I expect this happening when you are cropping. If you are taking a small part of the source image, and have the Crop tool set to width & height, then the area you are cropping from must have started at a smaller pixel size.
I'd suggest opening the source images individually, and cropping the area you want to use, and seeing what it's image size works out to. If less than your target size, either don't crop so tightly, or use a different image.
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I'd start by creating the new document, and going View > New Guide layout with these settings. That allows for the 20 pixel black margin in your image.
Then go File > Place Embedded and position and size using the guides like so
The drag out a selection which will snap to the guides, and add a layer mask by clicking on the layer mask icon at the bottom of the layers panel
That leaves you with a perfectly positioned image element. Note I have inverted the background layer colour for the black margin.
By unlinking the layer mask we can reposition the image without changing it's 'window'. Because it is a Smart Object, you can also resize it as many times as you need without affecting quality, but you still need to make sure you never make it bigger than the original.
Repeat the above for the other five windows. Job done.
Thanks for your detailed instructions, Trevor!
I will definitely try these and let you know how I get on