Syrian Hamsters are solitary and although they live together as babies and are often seen caged together in pet shops or at the breeders, as the hamsters mature their solitary instinct develops.
Syrian Hamsters will not, therefore, usually tolerate the company of another hamster once they reach approximately 6-10 weeks of age when fighting starts to occur. These fights may not be serious at first but as the hamster matures the frequency and severity of the fights increase. The fights most often take place during the night when the hamsters are most active and so often goes unobserved by the owner until serious injury or even death of one or both hamsters occurs.
Therefore Syrian Hamsters should be housed separately once purchased in order to prevent injury or death - the golden rule is one hamster, one cage.
Dwarf hamsters are sociable and will usually live happily in pairs or groups of mixed or single sexes of their own kind. Different species of dwarf hamsters should not be housed together as they have very different temperament and characters and do not inhabit the same areas or meet in the wild. Therefore housing different species of dwarf hamsters together is unnatural and results in an environment that is stressful to the hamsters and likely to have a detrimental effect on their temperament and health.
Dwarf hamsters are best introduced at a young age as an older hamster often will not readily accept a new companion, particularly if it has become used to living alone. When buying two or more dwarf hamsters to live together it is not necessary to get hamsters from the same litter but they should be roughly the same age and size (and sex if it not intended to breed them) and should have been living in a group community when bought.
Even when young hamsters are introduced or hamsters that have been living together moved to a new environment they may squabble and/or chase each other at first but the squabbling often sounds worse than it actually is and is just their way of establishing who is the boss in their new environment. During this process there may be much squealing and chasing but there is usually very little actual physical contact.
It is best to observe but not intervene during these squabbles to enable them to establish the hierarchy between them unless there is prolonged physical fighting or injury in which case they may need to be housed separately. Occasionally a very dominant hamster may not accept living with another but this is rare.