[chaos_alfa]

Originally Posted by Sansker

What about all the extra Assassins that were not Hassans?

Spoiler for Heaven's Feel:

Only Hassan is suppose to be able to be summoned as an assassin in the Holy Grail War.

Originally Posted by http://typemoon.wikia.com/wiki/Assassin

The system of the Fuyuki Grail War required originally that the Servants of the Assassin class must be one of the nineteen individuals known as Hassan i Sabbah -- the pseudonym shared by the leaders of the Hashshashin, a medieval Islamic sect. The name of the sect is the etymological root of the word "assassin;" and the class name itself thus acts as the catalyst to summon the members of the organization.[2] The requirement noted above was based in the rationale that those who might truly be described as perfect assassins would normally remain anonymous; and consequently fail to become Heroic Spirits. The wraiths (亡霊, bourei?, lit. dead spirits), the leaders of the Hashshashin -- Heroic Spirit Candidates (英霊候補, Eirei Kouho?) that might have been properly inducted to the Throne of Heroes if not for the loss of their original names[3] -- are thus regarded as qualifying in substitute of truly anonymous killers. With the corruption of the Grail during the third War; the absoluteness of this rule was compromised along with the disallowance of evil Heroic Spirits. Due to being selected from a group of Wraiths; their abilities are low.[4]

There does not seem to be any such rule in the Fate/Extra or in the Fate/Apocrypha version of the Holy Grail War; The Assassin in Fate/stay night , summoned by Caster, is an exception. He is an aberration in the system, as another Servant summoned him. Heracles has qualifications for all classes besides Caster; so it is unknown how he would fit into the role of Assassin.

Also, even if he wasn't a true Assassin he was summon because of something. Or the Holy Grail just lets anyone in to fill the spot if he is running out of time?

Caster used her magic to manipulated the summoning process which was why she was able to summon a (different) servant who was under her control.

He was a servant whose existence as a person wasn't certain. There where no facts to conclude if he had lived or was just a fictional construct, he didn't even know it himself. This type of servant was summoned because it was another servant who summoned him rather than a human master.