Hi issm81. Welcome to fishlore!
A 10gal tank is what I used to get back into fishkeeping. It is true that it doesn't offer too much options as for stocking, but still, if you do your research, you may be surprised what you can accomplish there.
If you can get a 20gal, I hope you do so keeping the 10gal you already have. That way you jump into
MTS (multi-tank-syndrome) from the start, which I encourage everyone with passion in fishkeeping to experience. You won't regret it.
It will be easier to do a fishless cycle. If I were in your situation I would return the fish to the
LFS, do a fishless cycle and make my mind while it completes (if done carefully, you could cycle both tanks simultaneously in 14 days, maybe 12).
Once your tank (hopefully tanks) is (are) cycled, bring your desired fish home, buckle up, and enjoy the ride.
Pepe
Santo Domingo