When you use Dialga GX's "Timeless GX" attack does Poison still happen in between turns?
Nope, the attack says to "skip the between-turns step", so Asleep, Poisoned, Burned, and any other between turn effects also get skipped."
Source: Ultra Prism FAQ (2018-01-19), TPCi Rules Team (2018-02-01)
If I use Arceus' "Type Switch" attack and choose a type, what happens if I use Ninja Boy to switch Arceus for a Basic Pokemon of a different type? Does the new Pokemon become both types or does the new type supersede the choice from "Type Switch"? Or what if you Ninja Boy for a dual-type Pokemon like Volcanion EX that is Fire/Water?
The type chosen by Type Switch overrides the Pokemon's actual type, so the new Pokemon would become the type chosen rather than its regular type. And in the case of a dual-type Pokemon it would become the single type that you chose for Type Switch.
Source: TPCi Rules Team (2017-12-21)
If your opponent used Sandshrew's "Sand Attack" on the previous turn, and you attack with Passimian's "Intentional Grounding" which requires you to discard a Tool from your hand or the attack does nothing, do you discard the Tool before you flip for the Sand Attack effect or after?
Flip for the Sand Attack effect first; if it's tails your attack ends immediately, or if it's heads you proceed with your attack as normal.
Source: TPCi Rules Team (2017-12-14)
If I use Xurkitree GX's "Lighting GX" attack to put a card from my opponent's hand face-down as one of their prize cards, should we mix up the prizes so he doesn't know which one it is or does it stay wherever I put it?
Rearranging the prize cards is not part of the attack, so wherever you put the new prize card is where it should stay until some other action causes the prize cards to be rearranged.
Source: TPCi Rules Team (2017-12-07)
If my opponent uses Sudowoodo's "Watch and Learn" attack to copy the attack my Pokemon used last turn, what would happen if I use my own Sudowoodo's "Watch and Learn" attack on my following turn?
Your Sudowoodo would copy your opponent's Watch and Learn attack, not the attack your opponent copied. So unfortunately your Watch and Learn would do nothing.
Source: TPCi Rules Team (2017-12-07)
Zoroark GX's "Trickster GX" attack allows you to copy an attack from any of your opponent's Pokemon. Does this allow you to copy an opponent's GX attack or not? If so, what if your opponent has already used up their GX attack for the game?
Yes, Trickster GX can copy an opponent's GX attack, even if the opponent has already used up their GX attack for the game. It does not count as using 2 GX attacks.
Source: TPCi Rules Team (2017-12-07)
When using Nihilego GX's "Symbiont GX" attack, can you place more prizes than your opponent started the game with?
Yes, you can. The prize pile is not limited to any maximum amount.
Source: TPCi Rules Team (2017-11-02)
If Cinccino uses "Amazing Plea" and there are zero cards in the discard pile, do I automatically do 80 damage? Or what if I only have one card in the discard pile?
The answer is the same regardless of the number of cards in the discard pile. Your opponent decides whether you take the cards (even if it's 0) or do 80 damage.
Source: Crimson Invasion FAQ (2017-10-20), TPCi Rules Team (2017-11-02)
Can I use Altaria's "Draco Melody" attack to place a Stage 1 or Stage 2 Dragon type Pokemon directly onto my Bench?
Yes, you can. Per the attack, you are not limited to Basic Pokemon.
Source: Crimson Invasion FAQ (2017-10-20), TPCi Rules Team (2017-11-02)
When using Mamoswine's "Forceful Tackle" attack, can I put enough damage counters on Mamoswine to knock it out? In other words, can I put more damage counters on it than it has HP remaining?
Yes, you can put up to 9 damage counters on it even if it doesn't have that much HP remaining. The extra damage will be done, and at the end of the attack Mamoswine will be knocked out.
Source: Crimson Invasion FAQ (2017-10-20), TPCi Rules Team (2017-11-02)