I can see 2 problems with this stocking:
1. H. formosa is really small, the females max out at 3cm, while Sparkiling Gouramis can get 6cm, meaning that the Sparkling Gourami might end up eating the younger livebearers.
2. H. formosa prefer harder water, since they come from the Southern US, while Sparkiling Gouramis come from Southern Thailand, and therefore prefer softer water.
a fish that I'd recommend for this is Elassoma evergladei, or any Elassoma species for that matter. They stay small, and ca handle harder ph.