Breaker size for 100 amp box
This should be fine

I would be comfortable adding the pool heater breaker (you'll need a HOM250) to this box; with only general lighting and receptacle circuits in it, I doubt you'll go over 100A here any time soon, and you have plenty of spare spaces here as well.  Also, you should be able to shut off the subpanel at the main panel, which means you can install the pool heater breaker and wiring with the subpanel completely de-energized.