About filters, a power filter may be a bit much. the shrimp are not very fond of fast moving water and power filters have to push a lot of water to get the same filtration of a cannister. I mostly only use air driven sponge filters. In one tank that is 20g I have a small cannister filter and I drilled a bunch of extra holes in a dispersion bar and put it on there so that the return water doesnt blast back into the tank. I am not too up and up on my conversions but 20L is a pretty small tank...less than 10G I think? Its your money, but in your position I'd get a cheapo air pump and one or a couple of the sponge filters that you can use an airstone with (I got one about 6 months ago and have since tossed all my 'big bubbles' sponge filters in favor of them, the airstone ones work a LOT better IMO). That being said, if you already have the power filter and a prefilter, you could give it a shot, I doubt it would kill them.
As far as why plants...They're pretty lol. Seriously the shrimp do seem to like them, they comb over them and stuff for algae and microfauna. I have heard that faster growing plants will actually inhibit that bio-film. I only use java moss, java fern, marimo balls, and any anubias species, as well as some floating plants. Also, it gives the shrimp something to explore and do, they dont seem to swim around in the water column unless the have a clear destination in their little shrimp minds (not very often), preferring to crawl along and around things. Also, this is purely opinion based on observation...I think the shrimp like the anubias because of the big leaves. I think they hide at night because when I turn on the light in the morning, there are always a lot of shrimp perched upside down under anubias leaves like little bats and I dont ever really see them like that during the day. I think the reason shrimp and plants go together so well is that sun (or sun-like)light is important to the bio film that the shrimp eat and since you already have the lights, why not go with plants? Again its your money and all that being said I know people have raised happy and healthy shrimp with fake plants and no special lighting.
On the substrate, I have several tanks of shrimp, in the ones for shrimp that prefer harder more alkaline water, I dont use substrate at all. I tie java moss, java fern, and anubias to small chunks of coral rock and toss in some marimo balls to give the shrimp stuff to play on. The coral also is good for the hardness and Ph for them. This makes it easy to clean up after them and for them to clean up after themselves. They tend to shred 90% of any food you give them literally to dust while eating the first 10% if that dust is on flat glass its easy for them to eat. I worry that the dust would get down into the gravel and cause problems. I am paranoid of gravel vacing because of babies.
for the shrimp that prefer acidic water I use ada aquasoil amazon because it lowers the Ph and I'm to cheap to get an R/O filter.
whatever you do, I'd recommend getting a darker substrate. I have heard that they will somewhat try to mimic a substrate and their color will look more washed out on a lighter one and deeper on a darker one.
food wise, lots of places say they eat anything, and thats true, but they definitely will let you know what they like and dont like. Mine mostly only eat crab and lobster pellets grudgingly. Algae wafers get a luke-warm reception as do most sinking pellets and flakes. They like shrimp pellets better than most things though and their favorites are spirulina flakes and blanched zuccini, they go nuts for them (why spirulina flakes but not wafers is a mystery to me lol). I'd recommend experimenting with the food to get a feel for their likes and dislikes.
