No, they're NOT safe. Leashes and harnesses are never recommended for guinea pigs. Their backs are pretty fragile in the sense that they cannot bend backwards, which is why they shouldn't have wheels like hamsters, ferrets and rats do. If wearing a leash and they get startled and yank the leash, they can injure, even break their backs.
Also, guinea pigs are prey animals. They really don't like outdoor open spaces, even if you're with them. Please forget taking them out on a leash.
If you want to take them outside, you need a pen with a light cover over the top. The cover will make them feel safer, and if the pen is moveable, you can put them on different parts of the yard.
But your climate is like mine -- hot, muggy, with plenty of fire ants and other noxious critters. Guinea pigs can't tolerate heat above 85 degrees without the strong possibility of heat stroke, and even temperatures in the high seventies are uncomfortable for them. That means there are very few months when they can go out, anyway. I'd recommend just making their indoor living spaces more interesting to them.