Congrats on the aquarium. First thing is first, the aqua tech 5-15 is only good for a 5 gal
aquarium. I'd get something good for the 20-30 range.
Of course you could always run two filters, or depending on how much you want your water filtered, one would be fine (rare and select circumstances..)
You can put your plants in at any time, they greatly help with the cycling process.
Before we can recommend any plants, do you have a light bulb/lit hood?
-Well you can always add java moss, regardless. The cool plant is a beast and grows anywhere.
I'd say heavily plant your 20 gallon and go for some small micro schools with a center piece.
Maybe a pair of Apistogramma and 3 schools of micro rasboras like Chili Rasboras.
The downside of the apistos are you cant have shrimp, my favorite tankmates.
Shrmip keep a tank healthy running and rolling! They're so fun to watch and add character to a tank.
If you wanted to do a large colony of shrimp, I'd recommend either Ghost Shrimp or Red Cherry Shrimp. You could easily fit 50 of either in that tank.
There really are no centerpieces which will not eat shrimp that I can think of, so with shrimp you'd have to stick to schools.
I love rasboras, especially the plain 'triangle' rasbora you commonly find in pet stores. They school quite tightly and have great color.
I like natural looking tanks