I don't know that that is true. A couple of shrimps (especially in 75 gallons) put off little to no ammonia. For this very reason many a many of reefers have had things like ich, let their tanks go fallow, then had some ammonia issues if they tried to just dump all of their fish in after the fallow period.Just so OP can rule this out, it would take a miracle from god to have a tank setup in December be uncycled while also supporting life such as shrimps or CUC.
Assume nothing. Especially since the OP has yet to say what he has in his clean up crew other then shrimp. And has yet to say what kind of fish. If he was putting super hardy fish like clown fish and damsels in and they are dying in 24 hours, then I would be more likely to be persuaded to a disease. As both of those fish can generally live through a cycle.