Yamaha HS5 vs KRK Rokit 5: Flat or Flattering?

In an untreated room, usually yes. An equilateral triangle with your listening position, tweeters at ear height, symmetric side walls and the speakers away from the rear wall will change what you hear more than swapping between these two models does. The speaker placement calculator on this site works the geometry out from your room dimensions.

For most home rooms, yes, and often better than larger ones. A small room cannot support low frequencies properly, so an eight inch monitor in a bedroom produces bass that the room turns into peaks and nulls you cannot trust. Five inch monitors plus a headphone check for the bottom octave is a more honest setup than bigger speakers in a small space.

You need the first reflection points covered at minimum: the wall either side, and the ceiling above the space between you and the speakers. Untreated, those reflections arrive milliseconds after the direct sound and blur everything, and no speaker overcomes that. Two panels and a pair of corner absorbers do more than doubling your speaker budget.

In a small room, often yes, because a sub can be positioned separately from the speakers to work with the room rather than against it. It also introduces a crossover to set up and a new set of problems if placed badly. Treat it as a later addition once the room is treated, not as a fix for a room that is not.