Current Transformer properties
Transformer core saturation is relevant to the winding in which power is applied (aka the primary). Secondary load currents (ampere turns) completely cancel with primary ampere turns due to those secondary loads. Primary magnetization current remains largely constant and it is this that causes core saturation.
The rated voltage for this particular CT is, I suspect, the rated voltage for the insulation - I note that the insulation voltage test being only 500V is very low for anything that might be used on power AC. I doubt that it applies to either primary or secondary voltage.
You can usually use CTs at lower frequencies but magnetization ampere turns will rise proportionately with falling frequency so you have to be cautious here to avoid core saturation. Going a little bit lower in frequency (but not enough to cause saturation) can mean you have to lower the burden resistor to get the same accuracy because you want the "transformed" effect of the burden onto the primary to be dominant compared to the primary reactive impedance.