This question of filters is a good place to review the intended fish, as there are some concerns.
First, Danio margaritatus is worth a rethink. This species is best in a group of 20, and in their own tank. This will allow them to behave and interact naturally, which is very rewarding to the aquarist. The intended 29g tank would suit them to a "T" but on their own.
If you eliminate the above, the panda cories need some water flow, more than many other fish, so the internal filter is a better choice.
Gourami do not fit with either of these situations. They need very sedate tankmates to avoid getting nipped, and they dislike water movement.
The platy is not going to do well in your water, it will probably hang on. I wouldnot worry about the store water, as they probably have dozens of livebearers. And hopefully those buying them understand their needs. You can't buy all the hard water fish to save them.