The Lumia 520 is a 512 MB device, so it's far more likely that your app is crashing because of a memory issue than that there's something specific to just the Lumia 520.
As far as I know, the only control you have in terms of who can download your app are the requirements you can define for the application in the application manifest and they all deal with things like NFC, front/rear camera and so on.
What kind of exception is being thrown that's causing the application to crash? Can you post the stack trace and relevant code?