Hi,
I can't see the reason why to have 10 different speed settings for a small fan.
At 1/2 nominal rpm the air flow is also 1/2, but the fan total static pressure is decreased to 1/4.
At low fan pressure the fan won't do what it is meant for - cooling. It will just rotate without the desired effect.
I suggest to set the lowest speed to have the fan run without adding a cap parallel to it. A cap might be helpful to reduce fan noise caused by PWM control.
On the other hand using a big cap will almost null PWM.
Boncuk