Hey mate! It isn't so much that you'll have imminent catastrophe, it's that you'll have an extra 5mm of space that'll cause your battery to be sliding up and down and means you won't have a real connection. e.g., if the battery's on the bottom, and your switch can touch it, the battery isn't reaching the positive post at the top, etc.
For 18600
batteries I believe there are spacers out there that'll convert it into an 18650 but there are only a handful of 26500 mods. There may be tricks with that particle model (longer springs, etc.) that I'm unaware of, so hopefully someone can help you out with a bump.