Multies are a very, very neat fish to watch. What they lack in color they can make up 1000 times in personality and the fun of watching them.
If you already have a 29 gallon tank, then it's fine to use. But most places I've seen, 29's are much more expensive than a 20L. Multies won't make use of the extra space a 29 will provide, however the extra water isn't a bad thing. In other words, if you've already got a 29, no reason not to use it, but if not I might save the extra money and get a 20L for these guys.
For substrate any fine sand will work. Don't worry about decor. They need at least 3 shells per adult fish, and they will move the sand around as it suits them. You'd be surprised at how much a male multie can move sand in a day!