It is often but not always possible to keep ship and Norway rats together. Introductions should be carried out in the same way as same-species introductions - see section on social behaviour.
Nursing Norway does will feed orphaned ship kittens (instructions on how to introduce babies to a foster mother can be found in the section on reproduction), although this is not always successful as Norway rats tend to keep their nest at too low a temperature.
Baby ship and Norway rats can normally be introduced together and will grow up harmoniously. This occasionally fails due to aggression by the ship rat: but it is safe to try it, as there is no risk of a ship kitten seriously damaging a Norway kitten.
Adult females of mixed species, or baby and adult females, can usually get on OK: I have successfully kept such mixed groups. For a while I also had two ship does living loose in my flat, and while they would squabble with any Norway does who happened to be out, I have also seen one of these ship does pal up with a Norway doe she barely knew, and exchange polite social grooming.
I would be much more cautious about introducing males of mixed species and age: an adult Norway buck might seriously attack a baby ship buck and vice versa. Introducing adult males of different species would probably be impossible on both sides.
Some Norway bucks do not recognize ship rats as other rats - or not as potential sexual partners, anyway - and chase them off regardless of sex. Indeed there are accounts from the Victorian era of wild Norway rats actually killing and eating ship rats, apparently regarding them as big mice - but the situation was very abnormal, with both species locked together in a small cage without having been properly introduced; and the Victorians loved blood-curdling stories about how vicious Norway rats are and may well have exaggerated. I have never heard of a case of a domestic Norway rat killing a ship rat. I have seen a dominant domestic Norway buck take a female ship rat's throat in his mouth and look thoughtful, but he didn't actually bite.
If you do get a Norway buck who recognizes a ship doe as a female rat, they can live together very successfully and have a passionate sex-life without the risk of offspring. Ship buck and Norway doe is both more and less difficult: ship bucks usually do recognize Norway does as female and respond accordingly, but they have difficulty reading a Norway doe's heat-cycle, and may pester her for sex when she isn't in the mood.
Very dominant and/or antisocial ship bucks may actually attack a Norway doe who intrudes into their territory - though the doe will just get scrabbled and scratched and generally rolled around rather than seriously savaged.