Will the default order "Load Fuel at Harvester" buy fuel from civilian harvesters?
No, but I will probably add a "Purchase Fuel from Civilian Harvester" default order at some point.
I have the same question for the conditional order to refuel from tankers now that I think about it. My civilian lines are beating me to deploying harvesters in my outlying systems.
Conditional orders won't buy fuel from civ harvesters either.
The joining of the civilian harvesters - could you add some kind of information on how much fuel each fleet contains? Would make it easier to see how many fleets I have to refuel from.
I needed this just a little while ago

In the v172 the civ harvesters will be listed in the Fleet Order targets list like so:
FH Lucas H4 083 (190k Fuel, 17% full)
FH Lucas H6 088 (890k Fuel, 14% full)
FH Lucas H6 129 (120k Fuel, 10% full)
FH Miah H3 054 (90k Fuel, 16% full)
FH Miah H6 114 (420k Fuel, 6% full)
FH Ward H6 001 (70k Fuel, 8% full)
I'm also moving them all below the player fleets so they aren't right in the middle of them any more.