You can get a pair from a LFS
or a local breeder if you know of one. You can pick two young clowns and the dominant one will become female. In order to minamize the aggression during the period where they are trying to figure out who will wear the pants in the relationship it is better to buy them with a noticeable size difference.
A starfish would depend on what kind of starfish you want to put in there. Some might be ok if you were able to suppliment their diet others will need a larger tank.
It is good to feed your fish a variety of foods. A mix of dry pellets and mysis are a pretty solid choice. Every once and a while you can mix in other foods as well.
The clowns will be happy in the tank you have planned out. There are not very many live plants for saltwater that don't just serve a filtration purpose like marcoalgae. Some people keep mangroves because they like the look but these are often kept in a sump or remote tank instead of the main display.
Most canister filters will work for a tank that size. I prefer them over a hang on back filter because they are a little easier to hide and tend to make less noise. A good canister usually has an easy place to put things like activated carbon or phosphate media as well.