Thanks for the great advice everyone
I have just realized that making 20L as sump would be very very messy. Because, initially, I thought that I can just half fill the new tank, transfer everything over there, make the sump, connect the sump and turn it on. But its going to be very risky to test the system with fishes in (and might be expensive in case I have to refill the whole tank with saltwater). So I guess I will LF sumps/ugly tanks.
Thanks for the input on ATO too. I guess I will use those round blue water jugs; will be easier to refill too.
And I will definitely increase the skimmer section (maybe 8"? because I found out that really big skimmers are like 10") and the return section. Just one question; the level due evaporation is going to change at the return section right?
Oh Oh! And how much are the baffles gonna cost me? I have no idea about those :P