In short : Don't worry about it.

You can alter the next extent size of some of the objects (like SOURCE$ and its index) if you want, but 100 extents is nothing to worry about.

Tablespace fragmentation is not really a problem unless objects are being created and destroyed all the time, and this does not (well, SHOULD not) be happening in your system tablespace. Just make sure NO user has SYSTEM as a temporary tablespace and only SYS has it as a default tablespace.

If you really want to prevent this next time you create a database, you can adjust the initial and next extent sizes of these objects in the create scripts.

-John