I'd like to use my own filters or Mosaic filters with DECam, even if that means obtaining images without the entire field of view.  Is this possible?  What would the reduced field be?

The location of the DECam filter changer is between the lens C4 and C3, whereas the Mosaic2 filter wheel was located right at the focal plane.  Therefore the telescope beam is much larger where it passes through the filters as it was when Mosaic2 was on the Blanco.  Using smaller filters would cause vignetting, reducing the star's brightness at the periphery compared to the image center and making the images impossible to calibrate.  One would need at least a 40 cm diameter filter to get any field unvignetted.  The Mosaic2 filters are 14.6 cm in diameter, so these are not suitable to use with the DECam.