Not sure what happens if you have no right leg/foot or have lost the use of the right leg/foot though.. Otherwise I strongly recommend using right leg only on first test drive of an automatic transmission car.. as said before, Don’t use left leg to brake coming from driving a manual only..
Due to my disability (polio), my right leg is very thin and pretty much useless. My left leg is much stronger/normal – and thus I have always used my left leg to drive an automatic car (I’ve only ever had an auto licence). I don’t have any adaptions – just normal pedals. I’ve never found it uncomfortable or weird because it’s what I’m used to.