Hello, 

As per your requirement, I would say that as you are awarded the scholarship of your degree under ews category so for applying for phd you need to have to apply for ews quota only. You can also opt for general category only as its a open category for all because you there is no reservations for general caste . But you cannot apply for under caste other than ews or general as others have their own reservation criteria. 

But its its always better, safe and advisable as to avail the quota which you really belong it as it create a fair chance and you don't get any seat of another student of general category as you already have ews quota. In favt, ews quota has its own reservation and some seats only for this quota so you can avail its benefits of ews quota. 

Good luck