It sounds like your ganache slabs are too hard and there are two possible causes; either the formulas are off making the ganache too hard when set, or your slabs (and perhaps ambient temperature) are too low, or a combination of the two. Ideally, you should be able to cut your slabs with no breakage or crumbling if your formulas are right-that being said, the slabs should be at room temperature, which should ideally be around 62-68F. The wires should go right through the ganache easily. But the ganache should be firm enough that you can pick it up with your fingers without damaging the edges. You'll know when it's right-wires cut easily, edges are clean with no crumbs/breakage, and you can still manage to pick up the cut pieces fairly easily without the edges being so soft that they squish from the slightest pressure from your fingers.
Chocolate foot should be down when cutting and very thin. I prefer to put the foot onto the silpat or sheet I'm laying my ganache down on first, then pour the ganache on top. Alternatively, you can let the ganache set, then add a thin layer with a spatula-but you get a flatter bottom if you lay down the thin layer first, then pour the ganache on top.