Unipolar motors have 8, 6 or 5 wires. If its 8 or 6 wire then you can wire it as a bipolar (4-wire) no problem. However the 5-wire motors have A and B winding centre taps connected together which might cause issues if wired as bipolar - although I think you will get away with it.
The darlington array is just a simple one-chip solution to driving a unipolar - bipolar motors can't be so easily driven. Personally I think the 8-wire configuration is the best - you can treat as unipolar, or wire as bipolar in series or in parallel - very flexible.
If you drive a unipolar motor wired as a bipolar you can get more torque and power out of it, note.