Relation between collector current and IF current in an optocoupler?
You definitely have to limit not only the collector current, but also the total power dissipated by the optocoupler, so you need an extra gain to obtain the 100 mA you need to drive the output transistor.

The easiest way is to interpone another BJT stage to get the needed gain.

You also need resistors to limit the maximum current flowing, based on the voltages involved.

The fact that you can drive the LED with 50mA only menas that you can get a deeper and faster saturation of the phototrtansistor, not necessarily a 100 mA collector current.