As something of a cheapskate (I think copper wire was invented by two of my ancestors arguing over a penny) I have gambled on both females being "pre-hit" only by males of that strain. Knowing my sources, I'm pretty sure this is the ticket. The larger female is swelling in the gravid spot and she is always hungry. She is even chasing the slightly smaller female away. They will have to be separated into separate (probably 5.5-gallon) tanks, put on the warm side of the room as November grinds on and I will still have to ante up for appropriately sized submersible heaters for both.

I know people in our area were getting clobbered last year by increased heating costs. It was even worse in some other places. And it probably will continue to be a huge drain on people's finances this year. (And around here someone in the Cook County government messed up the property tax lists. Billings were a couple of months late and so half-year payments will be inconveniently due Dec 3. Ho-Ho-Ho!)

Heating the tank(s) might be made a little easier by seating the aquarium upon a piece of styrofoam. Sometimes flat sheets come in the packing of packages. Perhaps if you know someone who works at a store or mailing operation, they might even be able to keep an eye out for you for expendable packing. It looks a little um, unusual, but one can even tape styrofoam sheets around the three sides of the tank. I'd put an aquarium backing or dark paper which would NOT run, if wet, inside the white styrofoam.)

Pulling shades, closing drapes - especially at night and situating the tank(s) away from windows, doors or drafty areas are other things we can do to keep aquariums a little warmer. A tight fitting aquarium cover is useful too. You probably do those things already.

During power outages, guppygirl suggested wrapping the tanks in blankets. I guess we are limited only by our imaginations and common sense. :)

I'm struggling to remember to close the door to the room where the guppies are. There is a cooler section of the house, which can otherwise influence that room.

Heaters are also obviously a concern here. For most of us, this may mean individual tank heaters. I wouldn't cheap out on them. Also, it is my somewhat substantiated prejudice that submersible heaters are more durable and efficient.

One of the things we have to deal with is correct sizing of heaters. Too small to meet the needs of a tank and they run and run and may burn out (or a switch may stick and everything gets cooked!) Too large and they may not go on frequently enough to keep temperatures from fluctuating.

In larger aquaria, maybe 20-gallon (or larger) tanks in cool rooms, it might be wise to put two heaters in the tank. The wattage for a 20 could be split between them. There should of course be a current from a filter, power head or at least an air stone to spread the warm and cooler water around and mix them.

Shirley Sharp at the About.com Freshwater Aquarium site has provided a useful chart matching heater wattage with aquarium size. Notice that a more powerful heater or heaters is/are suggested for tanks in roomer cooler at a certain point. So her chart suggests that a 10-gallon tank which is to be kept 1-5 degrees C or 1-9 degrees F warmer than the room should have at least a 50-watt heater. If the room is to be heated 5-10 degrees C or 9-18 F,  they suggest that at least a 75-watt heater should be used.

http://freshaquarium.about.com/cs/tipsandtables/l/blheatersize.htm

If there is even a greater difference in temperatures (10-15 C or 18-27 F) she or her sources (I think there is a typo on that page) should logically suggest that a 100-watt heater  be used. I frankly would worry about keeping fish in such a relatively cool room unless they were temperate zone fish whose water temps gradually dropped over the course of the fall and early winter. (Abrupt changes can be hard on most fishes.)

I do know of one guy locally crazy enough to keep a heater tank of common guppies in his garage! It is his electric bill and money!

That is clever putting new warmer water in. Don't let it be too much of a contrast though.

A summer time quick fix suggested here by miskairal was to put a drinking water bottle of cold tap water or even ice in the aquarium. It might be gentler to put a small hot water bottle in the tank near the filter outflow. But having to keep doing that so the temperature doesn't drop may be a tyranny you don't want to have to keep up with.

I sure hope that your young Moscow guppies are everything you hope for them. I admire both your persistence and that of your supplier. You certainly deserve to be delighted by the next generation of guppies. :)