I'm afraid I have tell you that the fish you have in there now aren't suitable for a tank of that size.
Both of those species grow to a good 2 inches and are very chunky; you'd really only be able to keep one of them in 17l; and they're both shoaling fish that need to be in groups of at least six to be happy and feel safe.
Tiger barbs are very aggressive when they're on their own and I wouldn't give good odds to either serpae or black skirt tetras in with it (those are two of the most aggressive tetras as well, btw). I'd worry about your cories as well; I've never known them to be kept with tigers with any success; the tigers always pick on the cories eventually.
I'd rehome the fish you have and have either a betta, a trio of honey or sparkling gouramis, or some male Endler's or guppies. You might get away with a small group (five or six) microrasboras if your tank mantainance is up to scratch.
Sorry to have to be so negative