How is Niantic going to introduce community day moves into the movepools?

Mewtwo has four possible charged moves. So does Hitmonchan. Mew has...I lost count. If Niantic wish, they can add to movepools without making existing moves legacy. Community Day exclusive moves are inherently not in the regular movepool for the featured species.

I don't understand the question.
Gen 4 may come with a moveset shuffle for existing mons like gen 2 and gen 3 did.
Starters are most likely going to get a community day which means Swampert and Feraligatr with exclusive Hydro Cannon.
Community days have their own code for handling the exclusive move, while other moveset changes are generally handled as replacements which creates legacy moves.

Doesnt seem likely, they may do a few legacy moves if it turns out new moves are super over tuned, but doesnt seem likely.

if anything, I think the most likely community days are Gible (duh) but the second one is hard to pinpoint, im not sure how many gen 4 pokemon will be 10k eggs AND have 3 stages, and we already know the starters will get the elemental hyperbeams

(and please, for the love of all things Arceus, dont say "gallade community day" because that cant happen, it has special evolution requirements in that it must be a male ralts with a shiny stone, so it wont be possible like gardevoir getting a double fairy set)