Dear SmallSqueak,
 
 >> Most Smalltalk classes build on a lot of other Smalltalk classes.
    
>    My English is not very good. Is there any difference between
>    "on" and "from".
 
 In this context, no real difference.

>    Is this a rule, that I have to add methods to existing classes?
    Would you please explain why it is a good thing.

 You don't have to, but you can if it suits your purpose.  Its a good thing because it allows proper delegation responsibility.
 
>    What if the authors of the existing classes don't want
    anything added to their classes?
 
 Adding something to a class only adds it for you and no one else.  Everyone is free to enjoy whatever experience they want, so you can add/change/delete code anywhere you want.
 
 Just like you can paint your Toyota a different color if you want, its your choice, and has no effect on Toyota or anyone else, so nobody cares.
 
 Just like for free software, anyone is free to modify it and repost it at a different location.  Others will either use it or not.

>    What would Squeak be like if no one(including the owners)
    were allowed to add anything to existing classes?

 Not "allowed"?  How would I be prevented?  
 Even if it were possible, I think it would stagnate.
 
>    I like to concentrate on just the cluster of the _smallest_
    core classes that they can exist by themselves with no
    dependency on any other classes.

 You might be interested looking at Craigs spoon project, which I think can start with a minimal "keyhole".
 
    http://www.netjam.org/projects/spoon/
 
 Welcome, enjoy!