6870 will not cfx with the 6550d igp. they're from two different classes. 6550d crossfires with only 3 discreet gfx cards - radeon hd 6450, 6570 and 6670.
when you add the 6870 and disable the igp, it'll work independently of the igpu. it's too powerful to crossfire with the 6550d anyway.
how do i know what is the max graphics card can be crossfire with APU?
another question if i installed 6670 X 2 crossfire it will work as triple crossfire?