20L tank is so small, you are better off using buckets to mix the salt water and if you use a tank you will just have to pump or siphon it into buckets anyway. You might as well just use a 20L bucket.
Anything will be fine to mix it, just as long as it mixes it.
Well my tank is only 136l, so taking into account rocks & sand etc, 20l is basically a 20% weekly water change. I have a spare top up pump as well, so figure I could use that to pump into the main tank, plus I think the 20l tank will just about fit on the shelf under the main tank so keeps it tidy and out of the way