Most pet stores are mis informed and just want to make a buck by selling set ups and fish. Kind of like baby chicks that are so cute and fluffy when little. I've known a few people who have bought a cute little common pleco... and then later learn they need about 125 gallon tanks and grow to 24+ inches long! For small tanks, a bristle nose pleco or otos are good, but do be careful that you don't get sold a common pleco or a chinese algae eater instead, as those get large, and become predatory respectively. It's very easy to impulse buy a fish but don't worry, we all start somewhere.
If your tank isn't cycled, might consider maybe returning a few in the mean time or keep doing lots of water changes till the water stats are level. The general liquid test kits that are about 30 bucks should last a few years and are great for keeping tabs on the water quality of the tanks. You can keep regular fish in those plastic kiddie pools outside if the pool is large enough and doesn't get too hot in the day. Some of mine live in out door pools.
They can survive in a small tank, just not the best for them and increases the work you have to do to keep them alive. It can also stunt their growth. Yes fish "grow to the size of the tank" as I've heard many a person say, but it's like wearing a shoe too small, your foot will fill it up and stop growing.
Here's an easy to read article on cycling the tank.
http://www.firsttankguide.net/cycle.php
I don't believe the "bacteria" in bottles work since with shipping and so on, they mostly die before you can get them in the tank. If you know anyone with a tank now, just ask to borrow some filter media and that can jump start the bacterial growth in your tank!
Edit:
Careful about guppies! They are live bearers and soon you'll have too many fry to deal with if the tank mates don't gobble them up.