Are you going to keep the parent fish in the five gallon?
Well, all the livebearers are gonna be easy as heck to breed, but they spawn so much you'd need another tank to keep all the cutie fry in. 5 gallons doesn't provide too much cover for vulnerable fry, much less a whole lot of room to grow. You could probably keep a pretty good ratio of male to female guppies in there.
I don't really know about the ease or difficulty of breeding anything other than livebearers....