If you waited one week before putting anything in, then you had to fully acclimate the shrimp. The water and temp would no longer be the same in a new tank after a week. If you had put the water in and moved the shimp at the exact same time, then you would not need to acclimate. Cleaners are amoung the most sensitive to any slight change and there is no way you could keep the salinity and alkalinity the exact same between an established 29 and a new 12 for a week. When you took water from the 29, you would had to have added new water so its already different. Then a week later plunk the fish and shrimp to a different setup? Shock stresses the cowfish and it emits a toxin to add to the difference in water chemistry. The shrimp died due to shock and probably some toxins from the cowfish. I wouldn't put more shrimp in that 12 until some stability in achieved and water changes are done. Kinda small for two shrimp it seems to me.
