As an appaloosa breeder, I stay away from the grey gene altogether. Nothing more disappointing than to have a well patterned foal born, that loses it's base color and turns white/grey everywhere, losing it's pattern. I prefer to not add the grey gene to the breeding herd.
As to size for breeding, what is the mare size, and the stallions that you are considering. The difference between a Shetland and a Falabella is quite a lot, generally. Shetlands tend to be taller, while Falabellas tend to be smaller. Not to say there are not some tall Falabellas out there, but generally they are smaller, and some are strong carriers of downsizing any foal they produce. But, of course, the first concern is the starting size of the mare, and the height of her prospective mates, IMHO.