I think a planisphere designed for 35 deg south will work just fine at 20 deg south.
The only think to keep in mind is that you will see the horizon slightly higher than it is indicated on the planisphere.
The best type of planisphere is a 'double sided planisphere'. This type of planisphere gives views looking south on one side and looking north on the other.
Edit: If your keen you can build your own double sided planisphere for 20 deg S if you follow the instructions on this site:-
http://www.asahi-net.or.jp/~zs3t-tk/planisphere/planisphere.htm