You can control Servants and Masters both if you wish. 

For the Servants I'll allow up to tier 3, Tier 1 being real figures (E.g Alexander the Great), Tier 2 being Legendary possibly real (E.G King Arthur) and Tier 3 being mythological, fake but believed (E.G Adam and Eve). Tier 4, being purely fictional will only be allowed if they are culture figures that almost straddle the line of tier 3, fake but ingrained over time (E.G Brothers Grimm Fairy tales, Sherlock, JB etc etc)

Of course I'll allow for certain will written loopholes for servants as well.