Guppies do fine in small tanks. I have no problem raising fry in drinking glasses, as long as I am aggressive with WC. A lone male or even a pair males should be fine in a well kept, unheated and unfiltered 2g. Just do a lot of WC (easy in such a little tank) and feed them every other day, which should be fine as without heat, they eat less, and grow slower.
Why is 100% WC bad? The snails and plants can handle being out of the water for a couple of minutes just fine, and I'm getting breeding from the snails and growth from the plants. If I had a fish or shrimp, or if I was using tapwater rather than water from an established tank, I could see why t would be an issue, but why is it a problem? I am learning, and would be happy to learn from you.