Hello See retainers are most of the time given after the treatment of braces so that there is no relapse or the teeth do not go back to their original position. It depends from person to person or the severity of case wherein the orthodontist decides whether to give a fixed retainer or a removable retainer or both of them are required. Since your doctor has told you to wear the removable retainers along with the fixed ones that might be the requirement of the case, so definitely just have trust in your doctor and go ahead with it.

Yes, of course.

Yes you can wear both at a time as both retainers have different function and they will help preserve the achieved correction.