An icon is worth having when it's a signal, which means it appears more than once and means the same thing every time. A row of five step cards each carrying a different icon is decoration, because no icon is being recognised, only looked at. The same five cards, in a deck where the magnifying glass always means diagnosis and the clock always means a timing risk, are a legend the reader learns once and reads for free thereafter.
The consistency rule has five parts, and a mixed set usually fails on the first two. One drawing grid. One stroke weight. One corner radius. One visual weight, meaning the amount of black in the icon. One metaphor family, so that a set does not mix flat pictograms with skeuomorphic drawings.
Why do icons drawn on different grids look different?
The grid and the stroke are where the arithmetic bites, because an icon's stroke does not stay the width it was drawn at. Take a set of 14 icons in which 9 were drawn on a 24 px grid with a 1.5 px stroke and 5 were drawn natively at 40 px with the same nominal 1.5 px. Place them all at 40 px on the slide. The 24 px icons scale by 40/24, so their 1.5 px stroke arrives as 2.5 px, against 1.5 px for the native ones: a mismatch of 1.667 times. Your eye reads that as two different weights of icon. Most people describe it as "these look bolder" and never find the cause. The fix is the same arithmetic run backwards. To match a 1.5 px stroke at a placed size of 40 px, an icon drawn on the 24 px grid must be drawn at 0.9 px. The same scaling hits every geometric property, so a 2 px corner radius on the 24 px grid arrives as 3.33 px. That's also why the small icons in a mixed set look rounder.
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Nine of the 14 icons need a redraw, which is 64.29% of the set. A mixed set costs that much to repair. So the cheapest rule you have is to take every icon from one family, drawn on one grid, and to check the grid before you download anything. After placing is too late to find out.
How should you colour icons?
Colour on an icon follows the colour rule from the geometry work: colour is a role. An icon set in one neutral, with a single accent reserved for the one icon the slide is about, does the job. A rainbow set makes every icon shout equally, and when they all shout, none of them is heard.