Most of these designs are things we saw in the movies. Some of them are things first introduced in the roleplaying game or expanded universe books. A few of them are just based off of cool pictures I found on the web. The latter were included mostly to 'fill out' sections that were otherwise sparse—either in number or in showing the true 'range' of droids in the Star Wars setting.
Some of the designs I changed completely. The dark trooper droids, for instance, look nothing like the ones in the video game (at least not the Mark I and II). And I omitted the Mark III entirely (since it was supposed to be a unique prototype—and I didn't like the design :P)
Some of the designs, I altered just a little—mainly to 'fix' things that never quite worked for me. Of note in this regard are the various Astromech droids. In this case, I switched the roles of the R3 and R4 droids. The reasoning behind this was purely functional—droids working on vehicles in an atmosphere should have aerodynamic, streamlined heads (like the R2 has). Thus, I gave the R3 unit (with a smooth dome) the role of vehicle droid (copilot/mechanic in racing or utility speeders) while the R4 unit (with a big, wind-resistant cone-head) works onboard capital starships. In my defense, the first time we see an R4 unit in the movies, it is a crewman onboard princess Leia's Tantive IV corvette (a capital ship).
Likewise, the predecessors of the R2 unit, the P2 and R1 droids, underwent a bit of a re-write in order to make their stories flow better. In MY version, the P2 provided the 'compact size' part of the equation, while the R1 provided the 'can function as a nav computer' part. Both of these inspirations then went into designing the R2. The 'official' story has the P2 unit being a HUGE R2...2 meters tall, but with all the gizmos and stuff that an R2 has. This just...doesn't make a lot of sense. It isn't as though Droid technology made a 'rapid advance' in recent years to smaller droids. Settings like Knights of the Old Republic (set 4,000 years before Star Wars) have small droids. I can't see that the technology could have back-slid that much.
So, yeah—there are actually a lot of minor adjustments and re-writes like this. Many of them come from having all of these droid designs placed 'side by side' in a single tome. I am trying to give each droid design a 'hook'—something that sets them apart from others in their class. Even droids with the same basic function can stand out from each other as something unique. This can be something as simple as a different personality or it can be an entirely different bank of programmed skills, emphasizing something that another design doesn't. The 3PO and LOM droids are a good example of this. The LOM is essentially supposed to be a copy of the 3PO, only with a bug-like head. This cosmetic difference would be, by itself, a bit boring. So I threw in a little blurb about the LOM droids having 'privacy' software installed (giving them the 'Con' skill)—allowing them to lie to protect the secrets or best interests of their masters. I have further extrapolated this software to be the reason some LOM units (notably the bounty hunter 4-LOM) have gone rogue (some technobabble about deception software sometimes corrupting the overall functions).
But one of the biggest challenges of this whole exercise is an attempt to standardize droid design (statistics) and to base the prices of the units off of some kind of coherent system. As I have noted on numerous occasions, West End Games statistics tend to vary greatly from book to book. One of the most prime examples of a WTF WEG moment was their stats for the K4 security droid. Here, you had a STOCK (and relatively affordable) security droid with a 7D dodge! This makes the droid nearly untouchable by most grunt NPC troops and even beginning player characters. And this isn't a 'top secret prototype' or something. So. Wow, yeah. Had to tone THAT down a bit. There are many other examples of this (though few as outrageous) that have to be smoothed out in my rules set.
As with all my source stuff, I am making a concerted effort to include a brief information blurb about each design as well. I do NOT want this book to turn into a just a huge list of stat blocks. After all, that's one of the things I dislike about the d20 Star Wars system. Therefore, with each droid, I give a little background on the design and how it fits into the Star Wars universe as a whole.
Oh, and in case you're wondering, yes, I included the Battledroids from the prequel trilogy. And no, I didn't include anything about them saying 'roger roger' or any of the other 'comic relief' crap they spout. I don't like that, and it's my book :P.