[ultasol]

First, many of the aquatic frogs they sell at pet stores get quite large. Another issue is the feeding issue, another is the small tank, and you haven't told us about your temperature or nitrate/nitrite/ammonia situation.
Please consider leaving your tank set up as is for a couple weeks before adding new fish. A tank goes through some water chemistry changes as beneficial bacteria begin to grow and colonize. In the meantime, the water conditions can be unstable and unfavorable to some critters. Use your betta as a cycling fish. The rule is one inch per gallon, and I would say your betta is a good 2 or more inches of fish. If you want a frog, get a seperate tank or return your betta. In with your betts you could possibly put a few (three) pygmy cories **once the tank is cycled**.

If you do things right and practice patience now, you will receive payback in the form of a healthy vigorous fish and a beautiful aquarium. Also, please remember that the LFS may not give the best advice. Not all LFS employees, even at small places, are well educated in all aspects of the hobby, or all types of fish.
If you listen to people here, you'll have such a good experience that you'll end up with a fish-addiction!
If you really want to add something now, you could try adding some plants, like java fern or java moss, or possibly some watersprite or floaters, instead of fish. Your betta will love you for it.
I occasionally have floaters available, and if you're interested PM me and I can send you enough for a five gallon for the cost of shipping.

Also remember your 1x per week water change (20-50%) with dechlor.

Also, stress defense/dechlor is not a way of removing ammonia, nor is it truly a way of preventing fish stress. Bad conditions will stress fish. Period. Nothing short of alleviating the source of the stress will solve the problem.