No smasher that should be kept in a 20 can break glass except G. chiragra. chiragras are boring anyways, so avoid them. You could go with P. ciliata, a spearer that can be ALOT of diff. colors and patterns, from bright yellow to red or green with stripes or spots. They are also very interactive, but I would suggest at least a 3" sand bed for them. Another mantis you could do is O. havenensis from sealifeinc.net but they are pretty sensitive and probably not for a beginner. Of course N. wennerae is always an option but it could be hard to spot in a 20.....Look through Roy's list (top of page) and see what species interest you.
Dan