Does the unit rate always have to be set up as the denominator?

Yes, as if you said your car was going 50 miles for 2 hours, and did this:

2 : 50

50 : 50,

it would equal 0.04 miles per hour. That is clearly incorrect. But, if you did:

50 : 2

2 : 2,

it would equal 25 miles an hour. Conclusion, you must have the unit rate set up as the denominator

Yes. It always has to be / 1.