You're not overly stocked at the moment, I think you can get away with adding guppies. I wouldn't add just three male guppies though. An all male guppy group is the way to avoid being littered with fry all the time, but you'll need a group of at least 5-6 male guppies to prevent a bully forming and picking on the others. Alternatively you could go for one male and three female, but as you probably know you'll end up with loads of fry!