a coilover set up uses custom springs with custom spring rates that allows you to switch between different settings. the coilover springs are also designed differently than conventional springs (a lot smaller than conventional, lowering springs.) it doesnt hurt to "mix and match" different coilover springs as long as you know what settings you wanna get and balance the front and the rear.
i guess generally speaking, if one particular brand offers springs and shocks, it's probably best to get that set such as in the TEIN case for the P11. BTW, you are confusing the TEINs with the term "coilover" the TEINs is a suspension system that's composed of lowering springs and adjustable shocks, NOT a coilover system.
From my knowledge, only Ohlin and JIC makes full coilovers complete with pillowball mount. the B&Gs do not come with the pillowball mount i believe.