First, welcome to Tropical Fish Keeping forum.
Filter. If you are intending plants, as you say you are, I would do an internal filter and something simple. A sponge filter connected to a small air pump works fine on planted tanks, I have this on my 10g, 20g and 29g tanks. Air pumps can be a bit noisy, so if this is a concern (a tank in the bedroom for instance) something like the Fluval U1 would be better; this is basically a sponge filter with the motor included, and it attaches to the tank wall in one of the rear corners.
On the fish, which gourami species were you thinking of? I'll maybe answer my own question, by saying the only really appropriate species would be the Honey Gourami, perhaps in a trio of one male and two females. They should (but no guarantee) get along with the bright-coloured guppies, and the latter might not get nippy. I've not tried this combo myself.
A 4-5 inch fish is going to be out-of-place in a 20g long with guppies. The Whiptail Catfish mentioned is fine, for its length it is not bulky. But largish upper fish will not look "right," believe me. And most of them will be problems in other ways.
Males only for the guppies, presumably; otherwise you will have hundreds of fry in no time, causing water quality issues.
Plants like Corkscrew Vallisneria would be good, if you have medium hard water. Some snails do not eat plants, unless they are decaying. The small pond snails and Malaysian Livebearing Snail are OK.
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