I half agree with coolall's advice. It's better to wait until you do the upgrade, then get the fish. These fish aren't rare; you'll be able to find one when you do the upgrade. Many (arguably most) planned upgrades never happen, or are significantly delayed.
That said, the reason I only half agree is that I think you need to wait for your next upgrade after the 75g, which is also too small. I felt my last one was cramped in a 200g. One of the issues there is that if the tank is too small, they might start eating things you don't want them to.