Then what corals are you planning? Do you have a list? We can probably tell you what is or isn’t feasible...
OP has actually said this multiple times. He wants to keep softies and macroalgae and possibly some low-demand LPS corals in a 20l. The corals he wants to keep are actually the most reasonable part of what he wants to do, he just has a not-terrific method of communication that makes confusion easy.