it depends on what you want to grow. If your new, and dont want to dose and get complicated SPS stony corals 6 might be total over kill. esp if your looking at lower light corals like many LPS.
common prob. folks dont match the light to the corals, and you cant really combine ALL corals in the same tank unless you have a really deep tank.
My reccomend would also be save a few bucks on the fixture and get an led strip for the tank, youll like the little bit of shimmer it gives and the Pop is friggin cool esp after the tubes go out.
No one likes the current orbit pro(cheap used), it hardly grows anythng. esp by its self. But I have one in with my t5 and man its cool. ramps up in the am real slow before work, corals wake up and glow.
same at night. I have a 2hr blue only at night. and man is it cool.
So yea, 4 t5 dimmable and a ramping led strip will probably really make you happy.