Question about planted aquarium and fishless cycling

Don't start a second tank just yet. It means twice the time/effort to spend on maintenance. You need to have the first aquarium for a few months to see how you like it first. While an established aquarium need minimal maintenance, you still need to do partial water change once a week. Some people might get tried of it after a while and skip on the recommended maintenance. Adding a second aquarium now will double the work and might accelerate the boredom (if) somehow you develop it.

If you add a second fish tank, might as well use it as a hospital/quarantine tank first. So it can be a smaller fish tank, and you can use it to quarantine any new fish you plan to add to your main aquarium. It is highly recommended after you have a well established aquarium, since it is easy to infect the whole tank if any new fish comes with diseases and parasites.

Guppies are also known for breeding really fast. They are live bearers (give birth directly to small fish fry). You definitely need some other fish in the same tank to control their population. Otherwise you might have your aquarium overcrowded before you know it.

Zebra Danios do need large tank since they are very active fish. They like to chase each other around in high speed.

By the way, what other plans do you have for adding new fish?