Heh you'll probably catch a bit of flak around these boards for using a fish as a coal mine canary.
Technically, it's a sound principle, but most consider it a mean method to cycle/test out a tank.
If you are indeed getting the bigger tank, the puffer would be find for a couple weeks in the small tank, provided the small tank is properly setup and maintained.
I'd really advise that you do your tank setups in parrallel as much as possible.
It really is necessary to get some bacteria going in whatever home your fish end up in, as those lil bacteria are excellent biological filtration.
Most people around here say:
Get the tank, clean and make sure it holds water properly etc - never clean with soap!
Put in your substrate, fill tank with filtered water (RO
) about 2/3rds of the way up.
Add live rock (it's expensive, but it is beautiful and FULL of life (and provides a great place for them bacterias!!!))
Let the thing sit for as long as it takes to 'cycle'.
A cycle is where ammonia is generated from the die off on the rocks (and can be supllemented with an uncooked shrimp from the grocery store).
The bacteria munch the amm, turn it into nitrites which get broken down into nitrates.
So a tank in cycle would test as a spike in amm, then nitrite, then nitrate.
Then it's safe to put in fish.
Now, you can do this with live fish, but it can really stress them out (to death) to live in these conditions without very frequent water changes, which will greatly prolong the cycle and equals more work for you and more stress for the fish.
I set my first tank up a little while ago.. got it in november.. it sat running with nothing but rocks for 6 weeks or so... but patience paid off and the tank is in great shape and my recent livestock additions are happy and apparently healthy.
Do not rush into this, it costs too much money to do it 'quick n dirty' like..
My 2cents...hope things work out well for you