You should probably only get the cleanup crew and one clown. This wil keep your bioload to a minimum. Also, how long has it been since your tank cycled?
If your tank is still cycling then no. When your tank is cycled, you should only put additives in your tank unless you really need it. Otherwise you will end overdoing it and kill the livestock in your tank.
Usually fish of the same species are ok. But sometimes an aggressive clown will attack the other fish. But usually this doesn't happen, unless put into a small tank. If the fish is aggressive, then bring it back to your LFS.
All fish, even of the same species, have different colorizations. Just like people have unique traits as well. As for my perculas, one became dominant and a mated pair within 3-6 weeks.
I am glad to say that the cardinal is eating the mysis very happily.
Thanks Bang for your input, I would have never known to have gotten the mysis w/o you!:)