If it's bipolar only 4 leads are connected to the driver. If the driver is a chopper (Xylotex or Gecko), then the total current needed is 2/3 of 1 phase x # of motors which is about 6 amps total. If not a chopper driver than you might need the 18a. Look at the White papers at
http://www.geckodrive.com for an explanation.
Drivers are rated per phase. 3a/phase motors wouls need a 3a driver.
Not entirely sure on the last one.