As far as protein I would go no higher than 20% (been there done that on going higher and you start having scratching, hairless and kidney issues) the fat content I try to go no higher than 10% the other important factor to consider which is present in a lot of dog food is the red dye, it is not present however in doggy bags.
You can use something specifically designed for feeder breeder mice such a Mazuri or Harlan however pet store mice food is just junk through and through and cannot sustain a feeder colony appropriately (pets yes but not breeders)
Is it ok to feed animals with tumor? Yes (I am pretty sure a wild snake would not get pass on any prey), personally I do not, when it happens and it is rare I just feed wildlife with them (then again I live on 4 acres we have owls around, possums, raccoons, skunks etc)