Here we are, another round of Tau rulings from the Frontline Gaming guys for their ITC tournaments. Let's just skip the intro and dive right in.
Models in the Piranha Firestream Wing formation may not leave the table using the Rearm and Refuel special rule the same turn that they arrive from Reserves or Ongoing Reserves.
No. Page 187 of Mont'Ka does not mention any limitation about having come in from reserves that turn. This does not even need clarification or eratta. Again, where's the in-depth playtesting and evidence showing that this likely unused formation is anything more than a laughable gimmick instead of something that needs to be nerfed?
When returning to the table using the Rearm and Refuel special rule, the Piranha unit does so at full strength, including regaining Piranhas that have been destroyed earlier in the game. However, models that have formed their own unit due to being immobilized are not replaced.
Meh, this could have gone either way, and I'm honestly surprised you are ruling that destroyed Piranhas get to come back. The rule on page 187 only states they return to play "at full strength" and unfortunately that is not a game term. Something being "full strength" is not defined in the 40k rulebook or the codex and of course thus it needs an eratta or clarification. I agree with this ruling, but I wonder if you just made a call here or if you think "full strength" clearly means model count too.
All Ghostkeels in a unit activate their Holophoton Countermeasures at the same time.
Holophoton Countermeasures, Page 122, Tau Empire Codex
"Once per battle, in the enemy Shooting phase, a model equipped with holophoton countermeasures can disrupt the targetings systems used by one enemy unit that is targeting it or the unit it belongs to. Declare that the unit will use the holophoton countermeasures after the enemy unit has chosen it as a target but before any hit rolls are made. The enemy unit can only make Snap Shots in that Shooting phase."
The word "unit" is singular as well as plural. If you have one Ghostkeel in a unit, that unit is using its holophoton countermeasures but it's a unit of one model. Single models are units too as defined in the first few pages of the core rulebook. When it specifies "a model can..." in the first part of the holophoton countermeasures rule, there's no room to imply that somehow translates to "all models simultaneously disrupt...". The first sentence seems to clearly define that a model can only use it once per game (not a unit), and "unit" just refers to the fact that Ghostkeels come with two drones automatically.
If a Stormsurge that has deployed its Stabilising Anchors is Tank Shocked, it must Death or Glory in response. If it fails to stop the Tank Shocking vehicle, it suffers D3 wounds and the tank is left in base to base contact with the Stormsurge at the point it made contact with it.
Well that's once again just reprinting the rules from the rulebook with your re-wording which is ultimately unnecessary:
Unstoppable, Gargantuan Creatures, Page 71, Core Rulebook
"Any attack that inflicts Instant Death or says that the target model is removed from play inflicts D3 Wounds on a Gargantuan Creature or Flying Gargantuan Creature instead."
So yet again, another round of GW-esque "clarifications" and also downright nerfings. Tau lose again, but this isn't unexpected.