To answer some questions:
Senth Kiran is a Rebellion-era Imperial Agent. He's a scout and hunter who's found full-time employment with the Empire. Being an alien, he isn't treated especially well, nor does he particularly enjoy his work, but he has a "a job's a job" attitude about it. POFT2 Tie Pilot arms and legs, Storm Trooper gauntlets and boots, and "prune face"'s head and torso.
Regarding Leda-Mem... I always found it amusing the Jedi Council is made up of middle-aged to ancient males, and the females all look 20 something and either wear highly ornamental robes, or barely anything. Leda is an attempt to balance that out a bit, give character like Qui-Gon Jinn a contemporary. Characterwise, she was once extremely gifted with foresight through the Force, but upon feeling that the exterimination of the Jedi was close at hand, she refused to believe her our instincts and blinded herself to the future.
The Fallen Jedi is actually a Wind Commander "Pilgrim Traitor" body, arms from the Rancor Keeper, boots from an Ep1 Naboo Pilot, hands from Jedi Training Obi-Wan, and the head is the horned Cantina Alien. An accomplished master of the lightsaber, he turned from the Jedi Order when Vader began to exterminate them, using the Dark Side as a sort of cloak or shroud in the Force to hide behind, knowing Vader's and the Emperor's attention would be focused on the Light.
SS-N8 is mostly rearranged Battle droid parts, the guns are from Xyber-9 weapons, the "toes" are from a medical droid, the backpack is from a Gundam, and that little bit connecting his torso to his hips is from an Avenger's accessory. The rest, as I said, are battledroid parts cut up and reordered. (reversed head, legs for arms, hips for feet, etc)
Originally, he was going to be partly transformable. He'd walk on 4 limbs, then turn his head/torso around, flip his limbs, and have a standing/firing stance. Well, that got dropped along the way, but when I posted the original pictures, Master_Phurby noted he looked a little like the striding creatures from Dark Crystal, so I took that last picture of him in the 4-legged stance... that's how I've got him posed now.
Storywise, he's an ancient war-droid prototype that ended up fighting FOR the Republic in the clone wars, against the droid army. Old EU says the Empire banned assassin droids and the like, so SS-N8 was forced into exile, and ended up working alongside the Rebellion from time to time.
Thanks for all the comments and questions guys!