Yeah, Ron pretty much nailed it with this: "The term there just refers to anything post-15Th century", a tosei gusoku in domaru form would be somewhat more form fitting then a pre 16Th century domaru, but that would be the only significant difference, besides that a domaru (tosei gusoku or not), would be better then a normal steel plate or hardened leather nimai-do tosei gusoku, assuming the materials and plate thickness are comparable, the lamellar armor (do-maru) would be more protective because of the immense amount of overlap with the scales.
Back in the early 16Th century they would often have their old style domaru converted into a nimai-do by inserting metal rods horizontally behind each row of scales in order to make them rigid, then they would add a vertical hinge on the cuirass under the left armpit, and so the cuirass would be made of two halves (back and breast) instead of one piece that wraps around the torso, a lamellar nimai-do provides somewhat better protection because the rigidity will disperse the force of the impact over a large section of the cuirass, a domaru would still have been the better choice for samurai who had mediocre armed combat skills but were good at grappling and preferred the flexibility of the domaru.
Ron also said something about the materials used so I will explain a bit about that: you often can't tell the difference between steel or leather domaru because they are lacquered, collectors are known to use a magnet to find out if it's steel, leather, or a combination.
Unless a lot of lacquer has come off, in that case close examination could be enough to tell the difference.
Besides that the weight of the armor is also a good indication of the materials used in it's construction.
Also, domaru made of different materials will have different descriptions in Japanese: nerigawa kozane domaru (all hardened leather scales), tetsu kozane domaru (all hardened steel scales), and the most commonly seen type: tetsugawa kozane domaru (a mixture of steel and leaher scales where the scales are arranged in an alternating steel/leather pattern), kozane= small scales (rectangular in shape, not fish style scales).
Finally, it should be said that there are tosei gusoku that were made to be shot resistant, and so these armors will always provide a better protection then pre 16Th century armor of any type (at the cost of 2.5 to 3 times the weight), so in the end you might say that the tosei gusoku provides better protection (if it is shot resistant teppo tameshi armor), however... a normal plate nimai-do does not
provide better protection then a scale nimaido or domaru.
I hope that was helpfull, and now for your second question...
A couple of decades after the Sengoku Jidai, in about 1625 to 1630, we see armor becoming more decorative, around this time it was the two nipple rings often with titty twisters... errr... I mean tassels attached to them, embossed armor also becomes somewhat more popular.
Later, during the early 18Th century, we see the appearance of extensively embossed plate armor, such a cuirass would be called uchidashi-do, the shoulder guards (sode) and the helmet would often also be embossed, here is an example:
The sode are signed: Myochin Osumi no kami Munesuke tsukuru (made by Myochin Munesuke, guardian of Osumi province), and dated: Kyoho mizunoto-u (1723). Interesting are the haidate that are made of steel scales made to look like mugwort leaves.
Besides that kind of armor we also see a return of the o-yoroi, a cuirass that has a separate armor flap under the right armpit to close the cuirass with, these o-yoroi were very decorative and of course created in tosei gusoku style (and thus form fitting), a very good example can be seen here: http://elogedelart.canalblog.com/archives/2009/10/25/15564222.html
Likewise, domaru with o-sode (large shoulder guards) also became popular again, these so called revival armors were mostly intended just for show, but are fully functional non theless.
Other armor types that became very popular during the late Edo period are the karuta gane tatami-do, made of karuta gane (steel card-shaped plates, hence the name karuta, meaning card), and the kusari katabira (Google that for pictures, they are made of mail), and then there was the kikko-do, made from hexagonal plates, an example of that:
Well... that should about cover it, how was that for some straight answers?
I will get to the other posts and the PM later when I have time, Cheers!