My humble observations as a guy with a printer who is rescaling models to fit his needs: It is easier to find good 6mm/10mm models and bring them up to 15mm scale than to take something meant to be big and shrink it. The good news is that there are a LOT of great 6mm models/epic scaled models out there that would look even better at 15mm. I've been doing this with spacecraft and the result is fantastic.