How wide should be the niche for a 29.5 inch refrigerator to allow the door to open? I have barely 30 inch wide niche. Is 0.5 inch enough?

It may depend on just how the door is hinged. Mine would be fine. An important factor is air circulation. The refrigerator must have air circulation to transfer the heat it removes from the inside to the air. My refrigerator draws air in the back at the bottom, over the coils, and out the front. Shove one like that back in a niche, and it may not get enough air in the back. Worse yet would be the ones with the coils in the side with both sides blocked. Your owners manual may give the clearance needed.

The niche is half open on one side, so there is no problem with air circulation. I was just worried about being able to open the doors.