[lagatbezan]

are you certain that its ich and not lympho? what was your qt procedure that you used before adding them to the DT?
Personally I would suggest you treat them all in a qt and go copper route if it is ich/velvet. The shrimp can stay in the main tank which should stay fishless for a min 76 days.
if you go hypo, you would need to make sure that the salinity in the DT stay constant and stable at 1.009 SG the entire time. Even a slight increase (caused by evaporation) and you will have to restart the treatment clock. you will need to make sure you are using a correctly calibrated refractometer as well. using a ATO would be ideal in keeping the salinity stable.
Also keep in mind that there are strain of ich that are known to be resistant to hypo which would make this whole process pointless if thats the strain you are dealing with.
You might get a ammonia spike by doing hypo so make sure you keep an eye on it and have freshly mixed salt water available for water changes. Ammonia badge in the tank would be great at monitoring ammonia.