|MadSci Network: Physics|
Lots of good questions here...
You ask how Einstein came up with the idea that light was a "maximum" speed. Actually, that wasn't his thinking at all: he was more interested in light being a constant speed in vacuum, for all observers, no matter how fast those observers were travelling relative to the source of the light. (The "maximum" you're referring to turned out to be a logical consequence of this simpler idea.) This concept came about (in part) from the Maxwell equations for electricity and magnetism (E+M), which essentially had the constant "c" in the equations, or the speed of light. If c was different for different observers, that would mean that E+M would work differently for different observers. Einstein didn't like that (and there was experimental evidence that it wasn't true). So as non-intuitive as his idea might seem, it make sense when applied to that particular context. (In fact, Maxwell's equations, developed before Einstein, were already relativistically correct!)
You're right that you can't draw too many conclusions about gravity from these postulates. Instead, Einstein built general relativity (which dealt with gravity) around a new postulate, called the equivalence principle.
As for the "rigid rotating disk" in relativity... you can read all about that here. Basically, there's so such thing as a rigid disk, because to stay rigid every atom has to know what every other atom is doing at that same instant. Because information can only travel at the speed of light, this is impossible.
Mass-energy (though not rest-mass) does depend on the frame of reference -- in both relativity and in Newtonian physics!
Finally, EM fields are a bit different from gravitation fields -- you can tell one from the other by seeing the difference between the acceleration on a positive vs. a negative charge. So you don't get time dilation effects in an EM field.
Try the links in the MadSci Library for more information on Physics.