FHM are slow swimming and easily preyed on... It'd be my guess 1 pound of FHM would last less than 3 days in your pond. GSH may not do much better, but have a far better chance at producing a self sustaining population. The larger GSH you can source, the better chance they'll take hold.
There are a few other native species you may be able to catch locally that could survive and do well in your pond. A couple are very predator tolerant. Their are a few species of topminnows(golden, banded and lined)they are first to come to mind and the best option IMO. They are very predator resistant. They will hug the surface but get much larger than gambusia. They all reach at least 3" in size. The flagfish is another option. You may be just a bit too far north for them, depending where exactly you are in FL. Another option is the sailfin molly. A livebearer like the gambusia, but it gets about twice as large. Another species found in your area that is fairly common in ponds is the taillight shiner. In my collecting trips in FL, I caught them in numerous ponds with heavy bass populations. They reach about 3" in size. Last, I'd see if I could source some lake chubsuckers, they would be the largest option of any of these species.
I am betting your gambusia heavily prey upon the eggs of your koi. If any of the eggs do hatch, they no doubt prey on the fry as will the HBG and others.
