The tank, as you noted, hummed along well.. You added the two Corydoras and the two mollies. They probably weigh a lot more than the two male guppies. Their waste material output, feces and urine - curiously mostly released with all the other exchanges going on through the gills, would be a lot more than the tank's biological system was "used to." In effect the bacteria processing the ammonia from the males would be too few and there again, as almost always, would be an ammonia increase with the additional fish. At that point your aquarium and nitrogen cycle seems to have been still working, but the limits were being pushed, though it might not have been obvious unless you were testing for ammonia. (By the way, I would be glad to be wrong.)

Even a lot of people, who understand what the initial nitrogen cycling process is all about, don't know or forget that every time new fish are added, that process must go through the same convolutions as before.

Now this is where my speculation is the most tenuous, but though the ammonia content rose, the Corys and mollies who were already there were somewhat used to that gradual shift in water quality. The guppy females, added a week later about at the probable new ammonia spike were not used to that dangerous water chemistry.

If fish are sort of accustomed to certain conditions, they can put up with a lot of abuse. New arrivals, already stressed by the process of being netted out of someone's tank, plunked into a plastic bag, put into a dark bag or box (though this actually is somewhat calming) and then acclimated to the new water chemistry (even if this takes an hour) may be shocked by the relatively impure water and different chemistry.

Several years ago there was an interesting discussion along this line on the Rainbowfish mailing list. (Rats! I can't find the original discussion nor a summary I gave here.) The basic point, confirmed by a number of very able aquarists, was that at a certain variable time (measured in years), one did not want to add new fish to "old" tanks because the darn chemistry of those old tanks was likely to lethally shock the new arrival. Old (3-10 year running) aquariums can be rehabilitated and their residents can be acclimated to newer tanks, though the process may take hours or even more than a day. Isn't it ironic that very new tanks (under 6 months) and long established aquariums are among the hardest to acclimate new fish to!

Your females sitting on the bottom is a classic example of nitrogen poisoning. I have carried on about this several times and you can get more on the topic of nitrogen poisoning by Googling Guppylog for that topic.

But basically what happens is that as the level of nitrogenous stuff in the tank (ammonia, nitrites or even nitrates) grows, it also increases in the fish's blood. Unfortunately the gills are awfully good at establishing equilibrium of whatever is in the tank's water and the fish's blood. These substances will prevent the hemoglobin from being able to effectively pick up and deliver enough oxygen to the fish. In effect they will slowly die of suffocation. That is why their movement slows to a stop.

This is also mentioned, in a different context, way down at the bottom of
http://www.guppylog.com/story/2006/5/20/16443/6233

Female livebearers, which are about to drop fry, indeed are breathing for more than one. They also weigh more than a male guppy. Therefore their oxygen needs are greater. I will admit surprise that the mollies weren't in that situation, though I'm sure glad they weren't.

I had a somewhat similar experience in an Endler's tank where one warm summer morning, I found four large and very dead female Endler's livebearers, a.k.a. Poecilia wingei. The smaller females, males and fry looked fine. The dead females, like yours, had no mark upon them. There were a couple dozen small fish in that ten-gallon tank. (I think it was Ginny Eckstein who suggested that that fish be called "The Endless Livebearer.")

The tank had no filter. I was using a layer of duckweed  (the world's fastest reproducing flowering plant) as a sort of ammonia sponge. As near as I can tell the duckweed became so thick that at night, when plants quit producing oxygen and actually reabsorb a little of it, the relatively thick 1/2 inch of duckweed cut off any significant oxygen exchange with the surface and the largest females suffocated. I threw out the vast majority of the duckweed and kept it limited. The younger females grew large with no more fatalities. Once my curiosity was satisfied that one could keep a thin layer of duckweed without harming females, I got rid of it all and replaced the duckweed with other greenery, which did not block the surface.

I would be willing to bet that a number of active and just viewing Guppylog people have had these mystery deaths when too many new acquisitions were added to an aquarium at one time. All of us have "eyes bigger than our fish tanks" and sometimes we get ahead of the biological processes when adding neat, new fish.

A good rule of thumb is to never more than double the fish population or body mass in an aquarium during any six-week period. Some people, using lots of plants, militant and frequent water changes and ammonia moving substances in the filters can get away with that - sometimes. The guppies also get away with that as they drop fry and the fry grow, but it is a very gradual process and the aquarium and beneficial bacteria seem to be able to adjust.

Hope this is of some use. Please don't add any new fish for a while. :)