Hi,
If you are going to use a Mega 2560 + RAMPS + Pololu-style drivers for the extruder + Marlin firmware, then the simplest route by far with the least waste of time and money is to follow Viktor's suggestion and just use Pololu drivers for all your steppers, and wire the small steppers as standard bipolar. You can find how to do that in the wiki.
As Viktor wrote above, you can adjust the current in the Pololu-style drivers to match your small steppers, this is also explained in detail in the wiki.
Good luck with your project,
Andrew
If you are going to use a Mega 2560 + RAMPS + Pololu-style drivers for the extruder + Marlin firmware, then the simplest route by far with the least waste of time and money is to follow Viktor's suggestion and just use Pololu drivers for all your steppers, and wire the small steppers as standard bipolar. You can find how to do that in the wiki.
As Viktor wrote above, you can adjust the current in the Pololu-style drivers to match your small steppers, this is also explained in detail in the wiki.
Good luck with your project,
Andrew