They won't fit! If, say, it's 1.25 instead of 1mm, you'll be trying to match 4 turns on the bolt to 5 on the caliper.
I strongly recommend you don't try, as you could very easily trash the threads on the caliper, and then it's helicoil or new caliper time!
You will not be able to use that nut. The pitch is wrong. You'd be in essence trying to force a coarse threaded nut (1.75mm pitch) onto a fine threaded bolt (1.5mm pitch). You will destroy both in the process. Either wait until you get the one you ordered or find one with the correct pitch.
You may be able to force the nut on the threads, but you can be assured of damaging and or destroying something. If the metals are particularly strong, you could destroy the spanner. That's the least likely circumstance.
More likely, you'll cut new threads into the nut or cross-thread the bolt by attempting to secure with the wrong size nut.
If the form of ...
You can't use just any nut that has the same size and thread, you have to make sure it has the same or better mechanical properties as the original, and by this I mean type of metal, grade of metal, locking/non-locking, etc. Suspension components take a beating, the most sure-fire way to get the right bolt is to order the oem part, which will have a part ...