Generally speaking, clowns are okay with other fish, but damsels are mean little fish. There are lots of stories where people get them early on and then regret it later. They are also tricky to get out, so I feel its best to go with other options.
Regarding the beach sand...there is no way I would do that. Too much potential for contaminants. Buying new sand is not all that expensive and this tends to be an expensive hobby. Skimping on stuff like sand and water makes no sense to me.