we spend most of our summer helping the keepers out ratting, used ferrets before, some ferrets dont like rat sets others love them, only ever had one not come back out, most the others come out with scratches etc. 9 times out of 10 the rats come off worse though, this summer weve got a dairy farm and its living with rats from top too bottom, i rate the smokers more than ferrets, especially when were working dogs but hay ho thats just me, 
got one this morning up me garden with the beddy whippet 1st cross b*****ds eat all me chicken food lol  
good luck on your new contract