If a 26.6 seatpost fits perfectly now with no play in it at all, then you need a 26.6 period. You should not play around with other size seat posts since all that will do is slowly weaken the seatpost and deform the seat post tightening ears. Your post should fit snugly with NO play.

A 26.2 post will be WAYYYY too small. You might barely get away with possibly using a 26.5 post but not sure if they even make such a thing.