FYI: Also using Biomes O Plenty (latest, but tried several recent versions) and Mo Creatures (latest).
Can somebody confirm that Zombie Pigmen don't spawn on portals? It seems to be an issue in forge itself as removing the mods didn't cause them to start spawning. I tried 9 max size portals in both Crag (BOP custom biome) and Extreme Hills (stock biome) to no avail. The issue appears for me in forge forge-1.7.2-10.12.0.1025 ... forge-1.7.2-10.12.1.1060 ... and forge-1.7.2-10.12.0.1061 (haven't tested any other versions).
I let 9 max size portals run in both Crag and Extreme Hills for an HOUR and got no spawns.