I have a five. Currently occupied by a male betta and a clown pleco.
I've used it over the years for neons, harlequin rasboras (when young/small), as a hospital tank, and as a place to put livebearer fry out of reach of the parents. Many mollies, guppies, and platy's have spent their "childhoods" there before "graduating" to something larger.
My kids have kept guppies (only 2 or 3) in 3-gallon Eclipses, so I would be comfortable with a few of those in a 5. Larger livebearers, probably not.