Do all micro sim s fit in the iPhone 4?

Hey sorry i'm a newbie with phones.

So my friend is selling me an unlocked iPhone 4. I'm going to Mexico and a cell phone company over there's going to sell me a prepaid Micro Sim for the iPhone. There should be no problem with it working right?
Or am I facing potential issues like the micro sim not working? I've never done this before.

If the phone is unlocked it should work. If it would be on the original carrier (which in your case it isn't) then it wouldn't. You should be fine!

The SIM card would fit as Micro SIM has standards as to what size it must be. Now if the carrier you would be getting the SIM card from is a Prepaid carrier then data services would not work but texting and calling will.