I'm a graphic designer at an offset printer. There's no required minimal distance between images at our company. You can overlap images if you want and we can print it. That said, if you are looking for advice on what will look good on a page and print the best.... I would lean more toward space between images of about 3mm. If you have an image and a block of text, use 3mm there too. What you want is equal space between the items on your page as it gives your audience the feeling that the information is organized (unless you are going for a more organic type of design). If you've done your whole layout already with 1mm of space, then go ahead with your work as it is.
For the exporting issue, the distance between the images should not change. In what way are you noticing the change? It it just by the way it looks to your eye? Have you turned on the ruler in Acrobat or Acrobat Reader to double check that the distance is different? Make sure that the ruler is also set to millimeters. You can also measure the distance with the "measure tool". (In Acrobat 10 this is located under View->Tools->Analyze.) If you find the distance is indeed different, try a different export setting in InDesign and see if you still have the problem. If the problem persists, restart your computer and try again. Finally, try throwing out your InDesign preferences and try again.