Simple switch with optoisolator - CTR related
In general, you should set the photodiode current to whatever condition the datasheet guarantees the CTR at. The Vishay version of the part (datasheet added to your question) specifies 10-20% ctr at 10mA, but if another vendor specifies 18mA as you've indicated, go with it.

The 10-20% CTR means that the phototransisistor will be biased to only allow 10-20% of the photodiode current, so yes, if the CTR is 10% and the diode current is 18mA, the transistor will only allow 1.8mA to flow worst-case. (You don't externally limit the current to 1.8mA - the phototransistor does.) Your 0.52mA load should work OK with the 18mA you've specified, just bear in mind that there will be a voltage drop across the collector-emitter of the phototransistor that you will need to factor into your design.