( "and on a four sectors AP, Do i need to change polarites between the sectors ?"
No. Not install the antennas too much near (about 1 mt between antennas it's good, You can made a X support).
X support that is a idea thanks, But till then i am thinking that to get a 1 mt between sectors, then i might mount the sectors diagonally to get that physical seperation, Ok a drop in height but i would rather avoid if possible any interference caused by sector proximity-
Network 100% MT for Now?