Yes ur device wont get affected because the charger is programmed such that when a 15W device connects to charging it will provide only the required power supply for the device... its the speciality of Samsung... And if u r much more interested in knowing abt the charging please search in google about PPS charging
Sry brother you can't use 25w charger , simply phone doen not support 25w fast charging . But you may use 25 w charger to charge your device and it is better to use the charger one which come within box.
