I updated my Rotface guide, but wanted to alert others what the "big oozes will now merge correctly" hotfix meant:
The Big Oozes will now merge to combine stacks, not cancel each other out. This means they will merge into one big ooze that's closer to exploding--or will explode immediately.
They acted in this manner originally on the PTR. When the wing was released last week, something bugged that caused both oozes to disappear when they merged together (something that many of the new, live-server kill-videos include), which enabled some guilds to quickly earn the achievement for having no big ooze explosions during their kill if they discovered it quickly enough and made use of the bug. They now merge the same as they did on the PTR, and though my guild did work last night to try to earn the achievement and got as low as 14% before one exploded, general consensus is that it will take much higher dps/gear before we'll manage it.
On a related note, if you want to get the Flu-Shot Shortage achievement on Festergut, I highly recommend everyone in the raid getting exactly 2 stacks of the inoculation, especially if you aren't a player with insanely high tank-level health. We had many of our raiders one-shot by the exhale when they only had one stack of inoculation. It will take a lot of coordination around spores to ensure that everyone gets exactly two of the 3 inoculations, especially if you're running two healers and they can't really move because they're sitting there spamming heals at a spiking tank, but it is definitely possible in current ten-strict gear.