there are no butterflies, tangs, or angels that could live in a group [6-10] in a 40 gallon tank.
a 40 is too small for one tang, but you could keep one dwarf angel in it...
*generally,* dwarf angels fight among eachother, especially among their own species...
I cant think of very many butterflies that could live in a 40 gallon... there are a *few* I suppose, but they would need to be on their own and need pristine conditions...
what about something like a pair of clowns? or a small school of chromis [~6?]...
I have recently seen a species of chromis that is half black and half yellow... very nice...
you also said "perhaps some rock or other decoration" ... most fish NEED decorations for caves, homes, and territory. and the majority of fish need live rock of some sort with an established growth on it...
IMO, you should concentrate on getting a lot of LR, maturing the tank, then a clean up crew before really thinking about fish to put in.
Drew