Hello and welcome to TFK,
I have to say, what your trying to achieve may not really be suitable. Like Jaysee mentioned 20gallons is small. This fish you want will just not fit. Lets take the Oscar fish for example.. Just one of those on its own needs a tank size of at least 75gallons according to the profile in the freshwater profiles section of this website top left of the screen.
most of those aggressive cichlids once they reached a certain size would just swallow those other fish you currently have in your aquarium. This may be the result your after, as you mention you hate your current set up.
Agressive cichlids, from what I know about them, require a particular set up, that creates the environment for them to thrive. One of the most important being tank size.
I think maybe some more research is needed, and/or a much bigger tank. your already pretty heavily stocked in your 20 gallon, and are not really providing the best situation for some of the fish in there. for example, the glass catfish should be in groups of 6 or more etc.
Others might tell you different, this is why this forum is so great. lots of ideas and opinions and you can pick and choose what advice you take on board.
All the best