The Pilot Mixable Color cartridges appear to be the same size as my Decimo and Falcon converters. However, the cartridge box reads, " This cartridge is exclusive for the Pilot Parallel Pen. Don't use it for other writing instruments." Not sure if that means the ink is not compatible in some way, or if it's just a proprietary thing.
I haven't tried switching cartridge colors with other brands, so the mixing effect may not be unique to these inks. However, The Pilot Parallel Pen Wizardry book recommends just placing one tip parallel nib down on top of another to mix inks. I haven't tried that, but the Parallel's nib design and proprietary cartridges dictates the Mixable Color line.
My one complaint is that the ball and washer design seems to be a problem when replacing these cartridges. If they don't reseat properly flow problems result.