For example, when the game start, can the first player put a stone on the starting tile?Yes, he can put a stone on the starting tile. More precisely, he put the stone on either the white group or the black group.
It is any number of micropul of the same color linked together by being adjacent to one another.
a) It is O.K. whether it's the same tile or not.Yes, it's ok if the group is on only one tile. In fact, a "group" of only one micropul could happen.