Silver mollies and silver dollars require an intermediate level of aquarium care. However, both require different aquarium water parameters. On top of this, behavioral differences make them poor tank mates. You should not keep silver dollars and any kind of molly in the same aquarium.
Silver mollies are a selectively-bred variety of mollies. Mollies include a number of species of central American livebearers closely related to guppies. Several species of mollies are available in the aquarium trade and hybrids are common. All have similar care, regardless of external differences, like coloration or fin shape. All mollies prefer hard, alkaline aquarium water. Additionally, adding some aquarium salt to the water seems to be the silver bullet to keeping mollies happy and healthy, though many other species of aquarium fish cannot tolerate this.
Silver dollars are a medium-to-large type of tetra. Several closely related species often show up in pet shops. These fish prefer to live in schools of at least five to six individuals. Their home waters in the Amazon drainage have very soft, acidic water, fed by rainwater and acidified by tannins from vegetable material. They are somewhat notorious in that many pet shops sell them as tiny juveniles to unsuspecting hobbyists who do not realize that different species of silver dollar may grow anywhere from six inches to longer than a foot.
The first reason you cannot keep silver mollies and silver dollars in the same aquarium is water chemistry. These fish prefer very different water chemistry in terms of hardness, pH and dissolved salts. Fish have adapted to the water parameters of their home range, and keeping them outside of these conditions can stress or kill the fish. So, mollies kept in silver dollars' ideal range will not thrive and vice versa. While mollies can survive in a variety of conditions from freshwater to saltwater, their preferred water always is hard and alkaline, water that would harm silver dollars.
Additionally, if either fish lives long enough, silver dollars may eat silver mollies. Some species of mollies may grow to about the size of smaller silver dollars, but this almost never is the case in the home aquarium. Most of the time, silver dollars usually will grow large enough to prey on mollies. Even fish with a peaceful demeanor, like silver dollars, will prey readily on smaller fish. For this reason, you should not keep silver mollies with silver dollars.