first of all, for selection of posting, there are many factors
the various arms come in turns to select the people they need and what they will see is your scores during your training, which will consist of generally knowledge, leadership, teamwork, etc etc.
of cos, if you show a ability to lead or instruct others will earn you a recommendation to be instructor but not necessary BMT.
peer appraisal is a factor but i doubt they will pick the lowest score as this means you are not a likeable person.
to many people, the whole process would look like a random draw. there are many theories about this and that but you have to know, nobody knows for sure anyone's actual score for training or peer appraisal. only the selectors know and like any job interview, i'm sure they will not purposely pick based on the lowest score of any factor.
for you. get into SCS first.