Battery charger mAh rating - Can it be ignored if battery type is the same?
Don't mix up mA and mAh! One is (charging-)current, the other one is overall charge.  

mAh = mA x hours, so if your charger charges a battery with 500mA for 1 hour, the overall charge transfered is 500mAh.
To charge the battery up to the full 900mAh will take \$ \frac{900mAh}{500mA}= 1.8~ hours\$.