Can you play ALTAR of FREEDOM in 15mm scale? Yes, you can. A game appendix discusses the measurement conversion and illustrates how I've based my own 15mm Fire & Fury troops on cardboard movement trays. Six infantry stands (24 figures) represent a full brigade of troops, which could range from 800 to nearly 3,000 men. A single gun represents multiple batteries, depending on the scenario.
But to be perfectly honest, I feel strongly the game looks and plays best in 6mm (or 10mm) scale. The condensed ground scale allows you to play the entirety of Gettysburg and Antietam on a 6'x4' table. By converting to 15mm, you will require larger tables, and even then, the ground scale looks quite distorted. The game mechanics play just fine, but it demands a great deal more visual abstraction. Rifle range, for example, will appear extremely short compared to the miniatures themselves.