So... I'm running 5-Str Medium DR Sunbeam Nickel rounds on the bass I have in mind - .045 .065 .085 .105 .125
Do I stick with same/similar gauge for flats?
Generally speaking, flats are naturally stiffer than rounds due to their construction (flat ribbon vs. round wire).
DR Sunbeams are more flexible than most rounds due to the round core.
So, it would be a good idea to go down in size when you're switching from the Sunbeams to a set of flats.