I agree with the tang theory. You can keep them for a little while in a smaller tank knowing you are upgrading but lots of people end up not doing it in a timely manner as life comes up with unexpected expenses and a larger tank might not take precedence. The yellow tang is probably fine. A lot of LFS
will sell you the small juveniles telling you it's okay for now. For now, sure, but in the long run I think you will find that you won't have the adequate room for its swimming habits. It's kind of like putting a Great Dane in a run and keeping him there. They need a lot of room to swim and while you may feel he has enough, in reality, you are putting them in a tiny space. Couple that with the fact that its shared space with other fish and it makes it even less. I would return them, or look online at Craig's list or other places to see if you could acquire a much larger tank. I know it's not what you want to hear but figure losing the money you spent on them over the next 2-3 years. Then think about the stress on them as they grow. Some might say "it's only a fish", but it's a living thing that counts on you to take care of it.