The Danio need a bit of swimming room, but they should be alright in a 30g. I think that setup (6 of each fish listed) in a 30g would be fine as long as you add absolutely nothing else.
Online during the winter is bad if you live anywhere that gets cold, or the packages have to ship through anywhere that gets cold (as in 60 degrees and below). You run serious risks as packages can sit for HOURS on a tarmac waiting to be loaded on a plane, even express mailed packages.
I would personally go with a HOB (hang on back filter) and a sponge filter, and probably get an air pump that had 2 air outlets and use one for the sponge filter and the other for a bubble wand or some such, the fish love bubbles and it is a nice visual thing. (Helps live plant growth if you are NOT using injections of CO2.)
Plants such as Anubias (pronounced Ann-eww-by-us), java fern, java moss, most cryptocorne species would work well.
Best of luck.
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