Its not the position that will select the player. Its the players who set the positions. Here, with sumon first down, and Nannu 2nd down, all Bulbul can play is 5 or 6. Handle chose 6. Most times with a good captain, you will see teams changing the batting order, Like sending a bowler as night watman or pinch hitter.
In this case, Sumon is definitly first choice. Then Off course Nannu.3rd down, Handle chose Hannan/Mridul sezan as they are openers, and should do good holding a partnership. Then off course you got Bulbul. Yeas, he could play number 5, if Handle wished.