Since all the bench planes - numbers 1 to 8 - are of pretty much the same basic design, differing only in length, width and consquently mass, you could in theory make any of them perform any bench planing task. There's no law saying you can't if you want to, either.
However, it's usually most convenient for most woodworkers to use a medium-sized plane with a coarse setting and a wide mouth for preparation work, a long plane with a tightish mouth and not-too-deep setting for truing-up boards after initial heavy stock removal, and a small nimble plane with a tight mouth and fine setting for final finishing and refining surfaces for fit or finish.
So in general - number 5 or 5 1/2 for heavy stock removal, 7 or 8 for truing up, 3, 4 or 4 1/2 for smoothing. Which one you choose is as much down to the scale of the workpiece and to personal prejudices and preferences as anything else.
But, if you fancy using a 5 1/2 as a smoother, go right ahead. Ain't no law agin it!