Pool size for sims can be adjusted in Register, I believe under Service. To Remove existing roles use Master Controller. Sorry, not in game atm but I believe you will find those settings in:
MC>Sim>Career>Remove Quit Careers>Homeworld and Sim>Intermediate>Career>
Castes can also be set up to more efficiently manage these things using Story Progression.
If you need help using or setting up the NRaas mods to achieve specific gameplay you would like please post in
Chatterbox over at NRaas. There have been many updates and enhancements to the mods over the past months and the members over there are experienced with the mods and will be able to help you further. There are also FAQ's and Tips available within the Mods pages.