Well imo class wise 60-79l-89 lvl brackets are balanced. Even WW Monk which is pretty bad in bgs at 100lvl is good at 60-79lvl
From my exp twinks prefer Mages/Hunters/DKs/Warriors/Rogues/FeralDruids at 70-79lvl. WW Monks and Prot Warriors might be best for 60-69lvl
As for Heirlooms i guess BOPgear+Gems+Enchants > Heirlooms without enchants/gems.
Blue gear from dungeons scale differently at 60-89 compare to 90-99 and 100 (90-99 also have alot twinks with crafted 705ilvl weapons and 715ilvl gear who one shoting 630ilvl geared ppl)
I mean Heirlooms and Blue gear are equal at 60-89, but Gear with gems and enchants counters them both