If there is a single living cell, you always _can_ use the yeast. I build all of my starters from exactly ten frozen cells and it works fine and dandy
In the question of whether it is a good idea depends on two things.
1) Sanitation. When dealing with small quantities of yeast, sanitation becomes insanely important. You want to be careful at all times, of course, but 100B healthy cells is going to fare reasonably well enough against small infections. When dealing with small quantities of dormant yeast, though, it's much harder. Sitting under an inch of beer, your yeast is still clean, but you'll need to be careful in how you harvest and wash it.
2) Number and viability. It's hard to know exactly how much yeast you've got under there. Mr Malty's pitching calculator is as good an estimate as you are going to get without a microscope. If you want to harvest and pitch that yeast, use the numbers suggested to you on the Mr. Malty calculator.
Either will work well enough. It's just a question of time vs. money.