As long as they were revered or worshipped and 'died' they will typically qualify even if they weren't all human. Like Fafnir ([s]Not Dragon Fafnir, Dwarf Fafnir[/s])could potentially qualify as a Servant... But as an abstract case as he's an 'Anti-hero' to counteract the Hero 'Sigurd'. 

So as of current which classes are open?