Good evening Charlotte! If my commander is turned face-down by some effect, do I get to mix it with other face-down creatures or I have to make clear what card is my commander?
You can never mix up your face down creatures under any circumstances.

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Good evening Charlotte! If my commander is turned face-down by some effect, do I get to mix it with other face-down creatures or I have to make clear what card is my commander?
You can never mix up your face down creatures under any circumstances.
If The Mimeoplasm copies anything nonlegendary, will it still be considered legendary? For Empress Galina or Champion Helm for example.
No.
Good evening Charlotte! Simple question: Silver Knight cannot be Bolted because protection from red. But can it be Skullcracked? The "damage can't be prevented" overwrite protection? Plus, Skullcrack "counters" Fog?
If damage can't be prevented, then Protection's damage prevention piece won't work, but Protection still stops Silver Knight from being the target of red spells in that case, so you'll need something more universal like a Blasphemous Act or Chain Reaction to kill it.
Good evening Charlotte! If I cast a kicked Rite of Replication in Reaper King, what happens at the end? I mean, I'm guessing that each King will see each other, so I could destroy 5+4x5=25 permanents, before Legend rule is apllyed and I have to choose one of them... is this right?
25 is correct.
Hi Charlotte! Could I "split" the mana of Generator Servant's sacrifice to cast two creatures? Both of them get haste?
Yes / Yes.
Good evening Charlotte! Player A controls 5 Insectile Aberrations (one of them is a clone) and player B has a Delver of Secrets. Player B casts Echoing Truth in one of the Aberrations, what happens at the end?
Insectile Aberration doesn't have the same name as Delver or Secrets, so only the Aberrations are returned to their owners' hands.
Good evening Charlotte! This one bugs me a bit: If I have something that lets me cast sorceries as instants (i.e. Hypersonic Dragon), can I use abilities that say "Activate this ability only any time you could cast a sorcery" (like Birthing Pod's) at instant speed? Thanks in advance :)
No.
"Activate this ability only any time you could cast a sorcery" is just shorthand for "Activate this ability only if it's your main phase and the stack is empty." It has nothing to do with when you can actually cast a sorcery, just when you can usually cast one.
Good evening Charlotte! Let's say I control Sarkhan the Mad and give him pro-black with let's say Faith's Shield. Then, I activate his first ability. He will try to damage himself, but he won't because it has protection from black. What happens? Is the whole ability countered, I can't activate the ability, or everything but the damage happens? Thanks in advance!
The damage will be prevented. The rest of the effect will resolve normally.