I recently purchased a slab home with in floor radiant heat. Slow heat up times coupled with fast heat loss & high heating bills make me wonder if the slab was properly insulated. I am considering relying more upon the backup forced air unit to heat my home while cut costs. My concern is that all of my plumbing runs through the slab. How low can I drop the slab temperature/in floor heat without the risk of freezing my water pipes? Could I ever rely upon only the forced air for heating - does that warm the slab enough to prevent frozen pipes - I'm guessing not. Also, I live in Minnesota where January highs are sometimes below 0.

Keeping the circulator pump running would probably provide the freeze protection you seek if you are not using the radiant system. Effectiveness of back-up system is dependent upon its capacity vs. structural heat loss.

That type of heat is very slow to heat up that is why you leave it run all the time.
In Minnesota the heat bills will be high no matter what you heat with.