Yes, 10 dH is usually considered the lowest GH for most livebearers, though mollies need it higher, 12 dH or above.
The Black Phantom Tetras are a better choice re water, but I would have a larger group, 9-10. I know this is not much space (I am assuming a 60 cm/24 inch length tank here ??) but they are not active swimmers. Some info in case you aren't aware... Floating plants are beneficial, as the species occurs in streams with floating and substrate-rooted vegetation. The substrate should be dark or the colours may pale; chunks of wood, and dried leaves, can also serve to darken the bottom. Sexes are easily distinguished; males are dark gray with black fins and a black shoulder patch, with an extended dorsal fin that can be unfurled like a banner above the fish. When displaying, the gray colouration darkens almost to black. Females are lighter, usually with a slightly reddish background colour, with a smaller dorsal and with vivid red adipose, pectoral and ventral fins. A ratio of 5 males with 4/5 females is best.