I am looking for a euphemism for "selfish" or "selfishness" (want to express it in writing in a workplace context to describe the specific behaviour of some colleagues, but I find selfish too direct)
This is not so simple.
To express what you seem to want, some creative construction will be needed.
Some of our colleagues are oriented to achieving their own goals. This is understandable and useful at times. But a common success will be all we can realistically hope to gain as a reward for our work. That success will not be except we work toward the same goals.
I think with some thought, the above construction can be worked to what you specifically need.