OK, let's do what we can. First general things. When you buy a test kit, do not get strips, they can be very inaccurate. The API "Master" kit (contains pH, ammonia, nitrite and nitrate tests) is one of the best you can get if they have them. No idea of cost in the US (I'm in Canada) but likely $30-40. It will last a while and has the tests you do need now and for a few months.
Ironically the undergravel filter may help as it provides a large area (the substrate), even though in planted tanks it is not recommended. But it can manage, so no issue there. I myself would not run the power filter with the UG at this stage. More filters is not beneficial in this, the bacteria need time to appear, colonize and multiply, and the number of filters has no bearing on this. The substrate is the best bed for them at this point.
Feed very sparingly. The more food that goes in the tank, the more ammonia will result, and initially ammonia should be minimal to allow the bacteria to establish. Alternate days for feeding is plenty, and only what they clean up in a couple minutes. Again, the less in, the less out.
More plants will help, there are not many at this stage. When you say "chemicals" do you mean any aside from the water conditioner? Some of these can cause more stress which will only worsen the fish's condition when dealing with ammonia and nitrite. The Tetra Aquasafe water conditioner is fine. By any chance did you get Tetra's SafeStart? If you did, use it
. This product is live bacteria and it works. If you don't have this among the chemicals, try to get a bottle Sunday; small size will do. This works to "seed" the bacteria. [Let me know what other chemicals you may have before using those further though.]
Daily 50% partial water change is good; once you have a kit for ammonia and nitrite, this may change to less often, but without knowing the level of ammonia/nitrite, better safe than sorry.
Now to the fish. There are some stocking issues here, thinking compatibility, but as you say the fish can't be returned we'll plod along with settling the tank. But something will have to be done about the fish once the tank is settled [are there other stores that might take them even for nothing in return?]. At least there is some space with 60g, and having plants and decor provides some hiding spots. And the fish will be stressed enough with the new tank issues that they may not begin to show their true behaviours yet.
This will get you going now. If I've missed something, I'm sure other members will spot it.