No offense to anyone but i never had any problems keeping guppies in smaller tanks, 2.5gal being smallest. I have bred guppies long ago and always keep some. I think you wouldn't have any problems keeping 3-5 male guppys in a 5gal, but it would have to be planted and have good filtration.
also there are different types of guppies like others have mentioned endlers, but wild guppies are also smaller about the same size, and can be just as colorful. Of coarse it depends on how experienced a fish keeper is too.
I just had about 5 male wilds living in what was a temporary 2.5 gal for a little over 3 months. I did 2-3 water 50% water changes a week.
I have been bit with the betta bug as well, but i can't give up my favorite guppies.