Guppies will be ok in there. I wouldn't do more than five though. If you want them to breed you wont have much color since you shouldn't have less than three females per male, so you'll be having three-four females and one male. Four or five males would be ok in there...although there's always the risk of tail nipping with bachelor groups, they usually get along without females around. Too many males, even just one extra, will likely result in stress and fighting, and the girls will be constantly pestered to mate, so be sure you think about it really well. =)
With endlers it's different. They're muuuch smaller and just as colorful, and the males tend to be more placid than guppies. You could have three males and three females, or just as easily have six males and 3 females. They also tend to not eat their own young. They're too small.