You can do all of this, it's documented in the application deployment guide :
http://docs.sun.com/app/docs/coll/1343.5
To specify a separate thread pool, create it via the admin console or asadmin, then use the <use-thread-pool-id> tag in the sun-ejb-jar.xml that you should include with your ejb jar.
If all you want to do is to limit the # threads that will be used by the bean then use the <max-pool-size> sub-element of the <bean-pool> tag, also in sun-ejb-xml
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