I've fasted with juice and water, switching off day to day as the mood hit. It's just fine, and actually prescribed by many fasting experts. I prefer juice fasting to any other method, however. I fast for long periods of time and water, alone, wouldn't cut for me.
Smoothies, though... Hmm... I would think that would throw your body out of autolysis which will leave you in a starvation state rather than a fasting state. Digestion is halted during fasting which is what causes your body to slip into autolysis. Smooties require disgestion. Even juice fasting experts recommend straining your juices after juicing
to make sure none of the pulp winds up in your system. I've accidentally caused digestion while fasting and wound up hungry and having to stave off the hunger yet again... Not fun.
If you can drink fresh wheatgrass you'll love it. Tastes nasty, I never could get used to the taste, but the boost of energy afterwards is great! It's also so good for you.
I'll be beginning my fast in a few days. Starting with a liver cleanse first (should be brutal) followed by the fast. Good luck on yours!