It will be an all out defensive borefest on Friday.Tierney was seen to be good enough to get thrown into important world cup qualifiers when he was 19/20 - against Slovakia, Slovenia & Lithuania out of position at right back (Patterson's natural position!), and at LCB against England a few days after turning 20.
WIthout the width that attacking wide players can offer, someone like Patterson is needed as a natural foil to Robertson on the other side who provides that width. Players are only ever ready when they actually play - at 19 Patterson should be playing if he's better than the alternative options, which he is. I agree with you that Gauld should be in the squad, but we also have plenty of options for the middle. If Clarke wants to shoehorn McTominay in at RCB where he can provide some ballplaying ability from the back (much like Tierney on the left), Scotland could be ambitious and go with a team like:
McTominay Cooper Tierney
Patterson McGinn Gilmour Fleck Robertson
With only one recognised true CB it could be dangerous, though with 3 CB formation you really need the players there to be capable on the ball to add attacking threat - and would be very ambitious/stupid to try against England for the first time, but for me that is the right mixture of technical and capable players that you'd be excited to go to games to watch play.
Mentality will be let's try and backs to the wall a draw so we can kid on we are still in contention for the last game.