Well, ideally you would have a cycled QT
ready and waiting when you bought your first fish. But even if it's not fully cycled it's still slightly different, because usually in a QT
are the main source of filtration.. ie through frequent water changes.
I would always advise against adding even the first fish directly to the main tank, because if it ends up needing treatment you will be putting added stress on it (and yourself) trying to catch it and getting it into the QT
. That doesn't apply to all fish, but most SW
fish are very difficult to catch. On top of that, now that you've contaminated the main tank you'll have to leave it fallow for 6 weeks before adding another fish. It seems like an awfully large risk to take.