I'm not one hundred percent sure what you mean by "posture principals".
I'll wait to hear what, exactly, it is you are looking for before I attempt to answer. I'm not sure if you're asking about martial application differences, or about form style differences or something else entirely.
In the meantime, I can answer the question of Wu Jian Quan and Yang Chengfu being contemporaries.
Yes, they were.
Wu Jian Quan called Yang Chengfu "Third Uncle", so close was thier relationship. They taught together at the same school for a time, I believe.
The main differences between Wu style and Yang Chengfu styles are in the "frame". Wu style is "small frame" Taijiquan, Yang Chengfu is "large frame" Taijiquan.
Wu Quan Yu was a disciple of Yang Ban Hou, but learned from Yang Lu Chan as well. The families originally practiced the same style, the small frame of Yang Ban Hou. It wasn't until Wu Chien Chuan subtly altered his form that they became "seperate" forms, retaining all the same martial qualities.
The movements of Wu style are smaller, originally to allow for combat in ceremonial court robes that were worn by the nobility of that time, but have since developed into a highly skilled means of "in-fighting" using small circle techniques.
One of the biggest problems I have had coming over to YCF style has been in staying "large frame". I tend to apparently "close up" my postures because I am used to being much closer to my body with my hands and my "open, rounded and extended" is not anywhere as visibly "open, rounded or extened", so appears closed even when in actuality it is not. My instructor is constantly pulling my arms more open in YCF postures.
Also, the "sit" is much deeper in Wu style. I often look in the mirrors at classes to see myself sitting way down in postures. I have to correct myself and come "up" quite often.
Next comes the difference in "single-weightedness" during the postures. The members of the Wu family I learned from were consistent in 100/0 weight differentions between the legs. They have a strict 100% weight and balance on one leg policy, no 70/30, 60/40 kind of splits ever in their forms.
You practice this constantly, by doing everything you do, whether practicing Taiji or not, on one leg at all times, completely 100 percent. The goal being to eventually be just as capable of doing anything on one foot that others do on two. This gives you a very powerful root through your positive leg and allows your negative leg to be free and ready to move at any time to any position.
I still practice this, no matter how unconsciously, to this day. I am rarely on two feet for any reason, outside of horse stance for third eye meditation or during the Nine Cuts of Saber, which are done 50/50.
The exceptions to this rule during the postures are "beginning", "single whip", "cross hands" and "conclusion" in the Wu form. These postures are practiced at a strict 50/50 weight distribution.
One visible difference between Wu forms and YCF forms is the touching of the fingertips of your left hand to the middle forearm of the right during Grasp the Birds Tail. I was told this was to close meridians in the arms and promote the more even circulation of Chi throughout the moves.
Also, no "giving back" of weight between the feet to make a transitional move. If you are in your 100/0 stance on one leg, and that leg needs to reposition to adjust for the next postures stance, you simply readjust that foot to the proper angle, without taking ANY of the weight off of it.
No shifting back and forth to adjust to the angle of your next posture.
This is possible, some have claimed it is not until shown otherwise. In fact, in many situations I have found, it is very desireable. Especially if you are very close to your opponent and don't have the room to make a "shift" or the leisure to "give back" some weight to your other leg in order to shift your stance.
I hope that's enough to get you started.
Let me know, exactly, what differences you were looking for answers on. I'll be happy to answer any questions I can.
[This message has been edited by Wushuer (edited 12-06-2002).]