With both MedStaff, Inc. and a company called Clinical One you do not have to have a roomate. With Cross Country and American Mobile you have to pay not to have one. It can be good if you have a considerate roomate. In fact, they get nicer places because of less expense.
Most travelers have horror stories about roomates and pay to travel alone.
I have also met older travelers who have not complained about their roomates and they are experienced top flight ICU nurses. To each him own. I think MedStaff, Inc. the only company I know, is good. With Cross Country and American Mobile you have to pay not to have one. It can be good if you have a considerate roomate.