Ive found that pre 60 is quantity over quality when groupped with 1 higher lvl, i.e. it would prob be better xp for the 58 killing 100 PoM trash mobs vs 50-60 NC mobs because of the major diff in hp = quicker kills. I dont think the xp diff per mob really isnt much from 50-61 vs 62-70.
CAn split the diff and try some tier 2 pop zones, PoD mobs die fast and the crawlers are the highest lvl trash mobs in that zone.
If you want to do the FD deal in my experience you have to root before FD to get it to work.
Crawlers in PoD do try and run at low hp, good chance to root/fd then break root with sk mob should still try and run = all xp for sk