If the stubs are not present in the classpath of the client JVM, then it will request them from the server, and the server will dynamically generate the stubs and provide them to the client. However since it is the server that is providing the stubs, you will be breaking the rule that no dynamic downloading of class files are allowed.
You may start the rmi registry programatically from within your server.
There is nothing in the instructions which forbids this.
There is the instruction "Your programs must not require use of command line arguments other than the single mode flag". Given this, starting the RMI registry from the command line can be problematic, as your codebase will not be set, and you may need a security manager. There are ways around these two problems, but why generate more work for yourself? Since you do not get extra marks for going beyond the specifications, and the specifications allow an easy solution, go with the easy solution! Hope this helps.