Well it's interesting to say the least. I have actually tried to acclimate guppies to SW with no success... albeit I acclimated them in a very short amount of time. Sometimes the point of something is simply to learn
I would be interested in some pictures. The only downside of this is the size of guppies would leave them open to being prey to a vast number of marine species. If you were really committed, you could try rearing the largest of each batch to attempt to make a larger more suitable guppy.