I recently received through the mail 2 Goliath Phlox plants, bare root and dormant. It is below freezing outside. How do I keep these plants healthy while waiting to plant, and should I avoid soil with fertilizer in it until the roots start to grow? Can I start them now in flower pots since spring is coming late this year?

You can start them in pots and quite possibly the sooner the better. I would guess that by the time you plant them, they awaken and grow, that could take easily 2-4 weeks!
Once they start to produce green growth, I would put them someplace indoors where they might get a little sun, otherwise a bright window would be good.
Fertilizer in the soil is fine because by the time you put them in the garden they will just be getting all cranked up and starting to make use of that fertilizer. I would think that in 4-5 weeks the outside will be ready for them and they'll be ready to go.