You could write a whole book on tadpoles. Here's a brief overview as I've raised a bunch.
The 5 gallon is a bit small. Tadpoles eat a lot and excrete a lot and that fouls the water. A larger tank would be much easier. Frequent water changes are a must. Treat them as you would aquarium fish. Do a 1/3 water change 1-2x a week with water that has been treated (dechlorinated) and is about the same temperature. Use lots of aeration with bubble stones and a good filter. If the tadpoles are small be careful the filter isnt too strong. In a small tank an old fashioned corner filter works well. It wont bother them and produces lots of bubbles which is a good thing.
Live plants are good. I used to use bunches of elodea since the tadpoles will eat it. You can also use boiled leafy green vegetable leafs like romaine lettuce, collard greens and spinache. They will also eat algea. Tadpoles love protein if they can find it. They will eat the bottom feeder food as you mentions. Another good choice is cubed freeze dried tubifex worms. Stick the cube on the side of the tank and they will have at it. They will not eat crickets at this stage.
Depending on the frog and conditions somewhere in the neighborhood of 2-4 weeks they should start to develop hind legs. If all is going well pretty soon you will see bumps where their forelegs will be. Once you see the bumps the legs should poke through within 48 hours. Now you have a frog with a tail. Their heads should start to change and develop more of a square frog-shaped head for lack of a better word. At this point they should still have a mouth that looks like a fish so they still wont eat crickets.
At this stage its time to change the tank. Lower the water level to about 6 inches or so. Continues to provide oxygen with bubbles and filters. You may see them take a gulp of air but keep up with the oxygen in the water. Now its time to provide some rocks and logs for them to climb out on. They probably wont eat at this stage since they are absorbing their tails. They may just lay on the top of the water not moving. Dont be alarmed, its normal.
Once the tail is gone they will start climbing up on the rocks and logs. They still spend a great deal of time in the water. Take a look at their mouth. When it starts to look like an adult frogs mouth they are ready for food. As in the mouth runs from eye to eye and no long resembles a fish mouth. At this point you can offer them approriate sized crickets.
Once they begin feeding and get a little bigger you can introduce them into their permanent home. You know the half land half water or 2/3 water 1/3 land or whatever depending on what you wanna do and what species you have. Most likely something common. Some kind of bullfrog or leopard frog. Big tadpole, big adult. If you have a tadpole thats 3-4" you got a big frog on your hands.
Good luck. Its fun to watch and not diffuclt. The main thing is keeping them clean as they develop.
And yes the girls at ths shop should not be holding the tadpoles. Its not even a great idea to hold adult frogs. Holding a tadpole out of the water is the same as holding a fish. They dont like it. A good suggestion would be to hold the clerks head under the water for a few minutes and tap on the glass. See how that grabs her.
Kidding...kidding...dont do that 
