Selection of worst case based on Solubility and Potency


When selecting the worst case based on solubility and potency. What does this mean? Do I select 2 worst case products or is it the least soluble product with the highest strength? Eg if I have Paracetamol 500 mg (slightly sol) and Sodium Diclofenac 50 mg (soluble). Which do I chose?
In this case I have chosen Paracetamol and test for paracetamol residue.

Now, with paracetamol and codeine. I still also test for paracetamol on the basis that if paracetamol is removed, to and acceptable level, codeine will also be removed. Is this a reasonable rational?