I realize that you guys are saving for the marine set-up. Space may also be at a premium. One somewhat satisfactory way of dealing with that is to get a 10-gallon over and under stand. Usually both tanks, with a little more air-line tubing and a valve, can be run off of one air pump.

Then you do a good gravel vacuuming of the ten you have in business. Save some new treated water for a couple of days. As you know, leave the tops open.

Then the day of the big shift, siphon some of the water from the established tank into the other one. Take a net and scoop up some of the gravel and put it in the second tank.

Actually if you have a clean bucket dedicated to things fishy, siphon a little water into the other tank. Then siphon the established tank down to maybe two inches. I have often moved 10-gallon tanks with two inches of water in them (but cracked a 20 with less than that).

Set up the stand where you want it to be. If you are using a multi-plug power bar, try and hand it on the wall at some height so that water will be less likely to splash on it or flow down cords to it.

If you guys really want to make things safer, see if a GFI (Ground Fault Interrupter) such as those in the bathroom or kitchen, could replace the wall outlets near your 10-gallons and also near where your future marine set-up will go.

Tank dividers are not fool proof usually. I have wedged a piece of glass into the gravel and that, short of siliconing it into a dry tank is pretty good. This hot, humid weather in the Midwestern US is about the best time to silicon things into a tank or tow fix leaks. But please let the silicone cure a FULL WEEK for maximum strength.

For a little more on that see
http://www.guppylog.com/comments/2007/3/8/225414/4935/2

A problem with all tank dividers is that if they don't rise to be snug with the tank top, fish and certainly this includes some guppies, can jump the divider. That includes the quick-fix use of an over-sized piece of rug mesh or the more expensive  professionally produced tank dividers.

An aesthetic factor is that the tanks don't look as nice with dividers. Keeping unrelated fishes on different sides of a divider is about the only 100% reliable way of preventing surprise births. Floating plants such as hornwort or water sprite will also calm (or block) some jumpers so that they don't clear that barrier.

Google searching Guppylog will get quite a few, sometimes redundant hits. Here are a few:

Tank dividers:
http://www.guppylog.com/?op=displaystory;sid=2007/8/13/51013/1588

Cracked aquaria pieces:
http://www.guppylog.com/story/2005/4/23/103451/639

Traditional dividers:
http://www.guppylog.com/story/2006/6/26/171958/359

If you do business with an independent pet shop, they might take really nice guppies in trade for dry goods. It may be that they have all of the guppies that they need. (Policy makers for big box chains aren't bright enough to realize that locally grown fish will be less stressed that fish flown in from around the world. Those fish are also far more likely to be used to local water supplies.)

Pretty soon we run out of neighbors and friends we can foist guppies upon (or we wish to keep those friends as friend.) Donations might be in order.

I have donated guppies and other fish to schools and churches (once to all of the interested teachers in an entire grade school district). Best is sending them with somebody you know. Since this is the start of a school year, taking a note to the school secretary and asking that it be posted on the mail room bulletin board or in the faculty lounge might work.

Since most public buildings are controlled by a preset computerized heating schedule, every classroom tank needs to have a good heater and probably a compete top, because the building may go down to 55 degrees F/13 C at night and over weekends.

From experience I learned that the submersible heaters are best. Though a little more expensive, they last longer, don't get broken as easily, are more efficient and most importantly don't have that little control dial tempting someone to fiddle with it. (And I guarantee someone will play with it!)

The note should probably offer the guppies for free. The teacher will pay enough for the rest of the set-up and increasingly these days that is out of their pocket. Leave a phone number. If you would prefer, arrange to leave a parent's phone number.

Somebody among your guppies may still become impregnated. Don't rescue the fry.

It may be that too many fry are surviving. You might wish to do what New Guppy Momma did with her 75-gallon tank. She found in a male dwarf gourami a fish which pretty much left the adult guppies along. However it certainly could and did take unwanted fry.

A recent book noted that in nature most fish are born to be eaten by other fish. I'm still thinking about that one too. But predation of most guppy fry in the wild is usually the rule.

If the population gets too crowded, perhaps because of an absence of predators, then the often inevitable alternative way of thinning over population is by way of an epidemic. That is an option you don't want in your aquarium. :)