I can't seem to get into the discord. Is it still up?
@WanderingDragon that should be fine, special humans with innate supernatural abilities.
Added: I imagine this universe has worlds and parallel dimensions.
1- You can choose your own servant. As mentioned in the Gatcha, it's totally optional. The main appeal is for people who don't know who or what they'd want to play, so they're basically asking for a random idea.
2- It is optional to do so. Yes. There are only four masters, and eight servants. You can make a servant but not a master, or a master but not a servant. But you're only allowed to make one of each. So you can't have two servants, or two masters.
3- You don't need to know too much about the franchise to get involved. General rule is "Bad guys are messing with history, so mages summon famous people to fight the bad guys."