See it's better to follow all the instructions given by your orthodontist because that doctor has given the treatment and these treatment can relapse if you don't follow your doctor's instructions so it's better to wear both as removable ones you yourself can remove while the fixed ones will be probably on the inner side so they won't be visible

Wear fixed and removable braces as per your orthodontist suggested.

You should get both the retainers. The permanent retainers will be placed lingually... So it won't be visible to other people. N the removable one you will need to wear as per the doctors instructions. Our teeth have a tendency to move back to it's original place after orthodontic procedures. Hence retainers are given to the patient so that after removing the brackets, the teeth get used to staying at its new determined space. Hence there will be no complications in wearing retainers. If it gets loose with time do visit your dentist again.

Fixed retainer will give you best results