Adding cycle and/or stress coat will do little for aiding the bio filtration is sustaining any heavy additions to the tank. If anything time and patience are your best defense.
I would say adding the two clowns would be just fine. Adding them together greatly increases the chance of pairing (although not guaranteed).
Even if the tank was fishless now and properly cycled, I would skip the tang. Not the best addition to smaller tanks or ones that have been newly set up. They rerquire much swimming room and a stable
After adding the clowns, if all remains stable, then look to adding another fish in a few weeks or a month. Moving to fast and/or stocking too quickly are usually the attributes of new tank problems...