If it was me i will call the pervious owners again, they know their bird wasn't healthy when they sell it, so it was their mistake not yours, if they don't have replacement then they should give you your money back.
Thats sound great, yes the dose will depend on the weight, i think for Safeguard its 0.4 or 0.5 per Kg, hens usually weigh between 3.5 to 4 Kg, so i usually give adult hens 2 cc.
The male should be heavier of course if he wasn't sick.