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Originally Posted by Mr. Guy

No, any hero can be summoned for any class... minus the one your list is missing (I believe). Say the current Saber was summoned into the war when another Saber was already summoned. She would then take on a different role, probably a Rider IMO. She would also lose whatever special abilities Sabers have and would gain Rider abilities (not Noble Phantasms though, as they are unique).

But since Saber is best suited for a Saber, chances are that she'll be summoned as one. I assume this is why all the heroes (minus one, you could say) are very much suited to their roles.

Spoiler for game spoiler:

Though I also read that the current Berserker could be summoned to any class besides Caster. So maybe there are some exceptions to that rule?

Hmm do you have a source that says any hero can be summoned for any class? Because, as you've mentioned...

Spoiler:

The current Berserker-class Servant for the 5th Grail War is Heracles, who is supposed to be "qualified" to serve in any class except for Caster. Upon reading that (from Moonlit World/EvoSpace's translation), I assumed that there was some qualification required to become a heroic spirit that was based on the level of combat ability in a certain style (presuming that all combat styles are covered in the 7 classes).