Applications on their own won't slow down OS9.
A packed hard drive, however, will. Its all indirect.
Don't worry about installing too many applications unless you suddenly have 3 megs of HD space left...
The only maintenance you really might want to do is clearing preference files for old apps every now and then. If you don't really know what you're doing and don't want to go around deletinge extensions, etc, then don't, because its really not necessary.
OS9 takes care of itself very well