Yes it is absolutely fine that they live alone, there is research out there that suggests that once the hamsters reach maturity it can cause them stress if they do not live alone. I think this is mainly because we can not replicate how they live in the wild. In the wild if they fall out, one hamster can just go off and remove itself from the situation, they cannot do that living in a cage, which results in injuries and sometimes death. Syrian and Chinese hamsters are 100% solitary animals. Dwarf hamsters can live together in certain circumstances but it is not something I advocate or recommend and it is more tricky than people realise.
I did a whole video about it if you want to know more:
Are Hamsters Solitary Animals? - YouTube
and the link to the research is including in the description below the video.