Welcome to Tropical Fish Keeping forum. Glad you joined us.
I agree with what's been said. Perhaps I can explain "why" a bit. Potentially large fish need space, a lot of space, in order to develop properly as they grow. Fish continually grow, and it is not only the physical space but the water quality resulting from that space. Smaller volumes of water more quickly become problematic as they fill with the bad stuff--organics, dissolved waste, pheromones the fish release--and this affects the fish's growth considerably. In too small a space (for the fish) problems with the internal organ development occur, what we term stunting; deformity, immune system weakening, and almost always a shorter lifespan.
Secondary to this, are problems behaviorally. Science knows that fish maintained in too small a space usually become more aggressive as they mature. With naturally-aggressive or territorial fish (such as the shark, gourami, angelfish) this usually means much worse aggression, but even otherwise normally peaceful fish can become aggressive. It is the fish's only way to lash out due to its frustration. Keeping shoaling fish (which are fish that live in large groups in their habitat, like your Glofish danio) in too small a group, less than 5, also cause this increased aggression. Things may "seem" OK now, but it is like the proverbial powder keg just waiting for something to explode it.
Other members have previously written of similar situations where all seemed fine, until one day the Red Tailed Shark suddenly lashed out and killed 2 or 3 other fish overnight. The stress leading up to this was immense. We must do what we can to prevent it.
We have fish profiles here, second tab from the left in the blue bar across the top of the page, and you can find most of the regularly-seen fish listed. Information on numbers (for shoaling fish), minimum tank size (only for the subject fish, other fish can increase this), behaviours, compatible fish, environment needs (rocks, wood, plants, light, etc) are given for each species. IF the name (scientific or common) is used in posts exactly the same it will be shaded so you can click on the name to see the profile, example Red Tailed Shark, Bala Shark. Please have a read.
If you can return the fish to the store, great. Some of them will be real problems in very short order. Then you can plan the sort of tank you want. There are as someone mentioned many options for a 30g. First off, what are your water parameters, hardness and pH, out of the tap? It is easier to select fish that will manage in your available water than it is to have to adjust the water for specialized fish, which is not always easy. You can find out about tap water from the water supply people, many have a website with data on the water.
Once you know the water, you can decide on the sort of habitat you'd like to create. A flooded forest, with lots of live plants and sedate colourful fish in groups; a stream with gravel and boulders, some plants, and stream fish; and so forth.