Vanessa,
Silver dollars get quite large over time the size of diner saucers. Maybe stick with four of just add one instead of two.
A group of corydoras or some a group of loaches, yoyo or zebra would add some pizzaz to the bottom of the tank, What kind of substrate do you have? If you did go with 6 Silver dollars you could go with some medium size tetras, they would school nicely and stick together because the silver dollars are much bigger and would make the smaller fish want to stay in tight formation. They would add color and activity.
A severum would go well like others said but I wouldn't do the tetras. If you get a medium cichlid as your "show piece" they could prove to be to boisterous for corys.