The honest answer is yes, the wise answer is perhaps not. I am temporarily holding much worse fish in a 55gal then a 2-3" Blue Jaw. However it's only advisable if you're 100% sure getting the larger tank. I have my 240gal sitting in the office, so I know the tank is mine, and hopefully will start being set up any time now. May is a looong time from now, and really anything can happen. Far to many times a set of circumstances keeps fishkeepers from setting up a larger tank, and they're left with a fish they're emotionally attached to, and can't bare getting rid of, but can't do a bigger tank, and the fish suffers.
If you wanted, you can sort of view the 55gal as a quarantine tank, and that's how I'd set it up, and just be an "extended QT." Once you get 125 up and running, leave the 55 going and continue QTing fish, so you won't have problems in the 125.
The other thing is the fish itself. Blue Jaw's aren't exactly remotely rare or exclusive fish. There's always a certain amount of exception with anything. If you happened to find Crosshatch triggers for $50, yea, I don't think most would give you too much lip for holding them in a 55gal. So there really isn't a great reason why you'd 'have to get' the BJ now, instead of when the 125gal is running.
EDIT; I'm sure you figured this, but obviously the stock would be modified from the original list, the Blue Jaw and maybe another fish or two.
