So, I purchased a silicone jelly case off amazon that they listed for 521 (at the time the only color was black and pink), but it is actually for 520. (Big surprise they lied!). But the case actually fits pretty decently (the black case). So even though the 520 might be smaller, the cases might fit depending on where you buy it from. I am not sure how to rate the case because one they lied, and two I wish it was actually more snug (and certain parts do not line up perfect in the back - camera is fine though), I literally just received it - so we'll see how it holds up during the day tomorrow.
So my assumption is the cases will fit, but the back covers will not.