Interfacing retriggerable oneshot with optocoupler
The 25mA is what you can run the 123 at without damage.  That doesn't mean the output voltage will not be reduced.  If you check the HC family characteristics you will find this graph;



It shows the output will be reduced 0.3-0.9V at 10mA and 25C.  You should increase the 320 Ohm resistor as much as you can and still have the isolator work.  If that will not work, you need a buffer.