The Shimano 105 pedals the OP is talking about are SPD-SL pedals. I do not know anything about SPDs, but with SPD-SLs to adjust float involves buying different cleats. For SPD-SLs you can tell by the color of the plastic "bumpers" (I don't remember the correct term, but they are the plastic parts on the cleat that protect against wear on the metal parts). The yellow ones have 6 degrees(I think) of float, whereas the red ones are fixed and have no float.

I myself have the same pedals as what the OP is looking at (PD-5610), and use the yellow cleats that came with them. They work very nicely