Is 20L tank big enough for a cichlid and plecostamus?

You can keep a smaller cichlid like a jewel or firemouth, or a convict, but big fish like Midas and Red Devils need much bigger tanks, at least 75 gallons - they get huge. Common pleco grow even more massive then these big cichlids and are unsuitable, while something that stays at a few inches long, like a bristlenose pleco, would work well.

I use my 20 Long for a pair kribensis, a lovely and active dwarf cichlid. Smaller fish means more space in the tank for them to explore and be active, while keeping them as a mated pair shows you natural social behaviours you miss out on keeping them solo.

You need a regular filter to suit the tank size - a little bit of overfiltering is good, but enough to blow the fish and plants all over the tank is clearly not. Canister filters are by far my favorate type of filter and all I use on my tanks anymore.

These cichlids do like to hide and explore how much so depending on the individual fish, though when you are in front of the tank they will just beg for food. Plants are great for them to hide and explore in, and caves are good for the same reason. By providing lots of decoration all over the tank the fish will be more active and there will be no fighting over space with the pleco.

Live plants are really about experimenting to see what works in your setup, but it is a good idea to do some research so you avoid the ones that need highnlight, complex tanks. Plants that tend to work well in simple, regular light with little extra care include cryptocoryne, anubias, cabumba, moneywort, hornwort, java fern, and java moss.

Not even close.

You're going to need a 100 Gallon tank for your Common Plecostomus.