The 15w fast charge can charge the 25w mobile phone, because the rated power and rated voltage of the 15W fast charge are smaller than the rated power and rated voltage of the 25W mobile phone, so when charging the 25W mobile phone with the 15W fast charge, it will only slow down the charging speed of the mobile phone, without damaging the mobile phone battery, so the 15W fast charge can charge the 25W mobile phone.
As long as the charging protocol is the same, it can be used.