3. NEVER USE PvP and PvE gear in one set in general
You need or full PvP Armor gear, or full PvE Armor gear. If you will have, for example, PvE chest but PvP legs, you lose expertise.
Lots of great information here, but this part is incorrect. While replacing war hero with Partisan/Conqueror armor, one piece at a time, my post-bolster expertise gain was exactly 1 expertise per piece. With full war hero (except the relics -- relics were matrix cube and light side relic), I had 2004 expertise. It went up to 2006 when I got a Conqueror blaster and Partisan scattergun, 2007 when I replaced an armor piece, 2008 when I replaced another armor piece, and so on. Perhaps it is true for people with higher rated stuff than war hero, but for old non-expertise armor from back when the level cap was 50, there does not appear to be any expertise bolster penalty for mixing sets, so long the PvP and PvE mods are on separate pieces of equipment.