No, you can’t combine inspectors.
You can open the Info popover via Tools > Get Info and tear it off by pulling at the triangle to float it. Then you could view both the floating and docked inspectors at the same time.
but I pretty much lose all of the metadata I had input in Paperless - again like Category, Subcategory, Amount Paid, Notes. Is this correct or is there away that metadata can be carried over?
That would be an issue of Paperless, not DEVONthink. Did you read this…