The black patch with yellow front could almost be considered a tuxedo colour form and the one that is pretty much black all over with a bit of gold is closer to a marble colour form. They are all the same type of fish just different colour forms.
Mollies need water with lots of minerals in and a general hardness (GH) above 250ppm, and a pH above 7.0.
Guppies need water with a GH above 200ppm and a pH above 7.0.
They are fairly peaceful fish that give birth to live young (the same as guppies). They should get some plant matter in their diet and if you can grow algae on the glass or ornaments, they will pick at that during the day.
With livebearers you try to keep all males or all females so the males don't harass the females. If you want males and females then keep at least 6 females (preferably 10 or more) per male.
Male livebearers have a long thin anal (bottom) fin, whereas female livebearers have a fan or triangular shaped anal fin.
In picture 89557 the fish on the right looks male, bottom fish looks female, can't tell what the one on the left is.
In picture 89556 the fish is male.
What other fish do you have? If they are all livebearers, adding seashells can be a decorative way to bump up the hardness a bit naturally, but cory catfish would not approve, they need water on the soft side.
Just be sure that you use ones either from the beach or sold as aquarium safe, some shells from souvenir shops can have paint and lacquers that are toxic to fish.