It really depends on what it is, exactly, and what the scales are. If you put a 1:24 scale model of one car next to a 1:25 scale one of another type entirely, nobody is going to tell the difference — but put 1:24 and 1:25 scale models of the same type of car next to each other, and someone will probably comment that, “One looks bigger than the other …” The greater the difference in scales, the less compatible they will be: a 1:72 scale tank next to a 1:76 scale one of a different type may or may not look off, depending on what exactly they are.
Figures, you’ll be able to mix and match to your heart’s content in 1:24 and 1:25 — only if they’re holding items with a definite, recognisable size should you probably make sure to use the same scale for that item on figures posed next to each other. However, this is more an issue for military figures than for civilian ones: if you’ve got two soldiers holding the same rifle, but one of the rifles is clearly bigger than the other, this looks odd.