Pretty much every single EGT gauge you can buy here in my country is in Celsius. I live in a metric country.500 - 650 degrees "C" is not at all very very low. It equates to over 1200 degress F and is very much near the piston melt down range.
My question is where and why did was a pyrometer that is calibarted in the metric "Celcius" sourced to begin with? I have honestly never heard of such an animal. I wonder if it really a gauge that is calibrated in "F" and has been miss-labeled as being calibrated in "C". That makes alot more sense.
Yes, 550-650 degrees is quite low and completely safe. Just remember EGT is NOT the piston temp.