Scuds and Shrimp can be kept together, however the scuds can eventually out-compete the shrimp. Short term they can be kept together very well, the scuds tolerate more extreme ph fluctuation and temperature variance and are more prolific for this reason and will likely outperform the shrimp breeding, so they will crowd the shrimp out eventually. But if you are regularly harvesting the scuds only this may work very well for you. The nauplii (baby shrimp) are hard to distinguish from the very young scuds. If you decide to keep them together harvest scuds that are large enough to be certain they are scuds.