Not sure what you mean by "water cycle process when I decomm" as water cycling is for SETTING UP a new tank to get the bacteria/water parameters right prior to introduction of fauna. (shrimps or fishes).
For the types of tetras, it's a personal decision - get what you like! For a small tank, you'd probably not want to have too many species as you'll need a minimum of about 10 for each species as they are schooling fish and need to be in a community. I'd recommend perhaps 10 of 1 kind only.
Most common and popular is the Cardinals, with the Neons a close second. Look up the requirements for the species you have selected and it should be quite straight forward to get it up. Plants will be good as it helps with the ecosystem and makes the tank more pleasing (IMO).
All the best!
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