Another way of doing this is to edit each of the photos individually (i.e. as separate photos on their own), make sure as Gary mentioned that the resolution is the same for all of them and for the Background layer. Dimension the photos individually for the size you want them to be in the final montage. Then do a temporary "Merge Layers" for each. Then create a new Background file and click-drag the finished photos into it, placing them using the move tool with the grid switched on for accurate placement. This way you don't need to create a hierarchy of clipped layers to their bespoke files and concern yourself with correct layer placement on the canvas to be printed. And you preserve individual adjusted copies of the photos without needing to save the composite. The downside of this approach is that if you then want to amend anything, you need to remove the photo to be amended from the montage, un-merge its Layers, do the amendment of that photo and then re-merge and re-drag it into the composite canvas. When finished, to preserve the Layers of each photo, undo the Merge Layers, Save and Close.
Frankly, the only thing I print out of Photoshop these days is printer evaluation targets that need Absolute Rendering Intent. Everything else I print from Lightroom. It's just so much easier and more seamless between editing and printing. I make composites in Lr all the time. What's really nice is that if when you put several related photos into the same canvas, and you see that for sake of certain kinds of consistency between them, adjustments need to be made to one or the other, in Lr you just flip back to the Develop module, make the adjustments, they convey automatically into the Print module and you're done. Now if you absolutely want to continue printing from Photoshop, so be it, not saying not to, but just suggesting that there are other, ostensibly easier ways.