The problem with knights is that they all work best as Saber. However anyone of them could qualify as Rider or Lancer given the nature of their work.
Maybe Pellinore as Lancer from his days of hunting the Questing Beast (which would make for an epic Noble Phantasm), but also because I love the version of him in The Once and Future King.
Also, I'm not seeing Mordred as an assassin.
I have to agree, he might have been evil, he certainly wanted power, but I can't see him as a sneaky and stealthy yet weak Assassin type. In the legends after Arthur ran him through with a spear Mordred pushed himself down it to kill Arthur. For Caster and Assassin you might have to give up on the knights and use other figures or make them very unconventional Servants.
A bit off the question, but can you imagine a war with Mordred, Arthur(ia), Lancelot, and Gawain? The fourth war would be a firecracker compared to it.