heh that's a really hard question to be honest, your best bet though would be to put the ferrets or kittens separately in a cat carrier so there is minimal contact but there is some, do not, I repeat for the sake of your kittens put them straight in together. after they have been around one an other maybe put 1 ferret and 1 kitten in a area which you can watch and intervene in case anything bad does happen, you want to be careful with this so take your time introducing them.