The absolute minimum size tank for a trio of guppys is 5gallons, so anything less is too small- ideally 10gallons is much better for guppys. You are right though that guppys are sociable fish, so at least a group of 3 (either 2females and a male, or 3females) is best, you should have 2-3females per male for a mixed gender group or a minimum of 3females for an all-female group or a minimum of 5males for an all-male group.
All livebearers need filtration (and preferably heating, to keep the tanks temp at the right temperature and to prevent large fluctuations in temp). You could posibly use the tank to raise some fry in for a short period of time if you get some filtration and heating in there, but other than that the only fish that can go in an unheated and unfiltered tank is a single male or female betta/siamese fighting fish. Such a tank would still need very regular water changes though as there is nothing to break down the nitrites and ammonia in the tank, which if left to build up will kill any fish eventually.
What do you currently have in the 6gal and is it filtered and heated at all?