Do you know how to use the addons folder? I think the simplest way might be to make an add-on to override your workers' XMLs to only allow them to build the buildings you want, and override the buildings to only produce the units you want. You might also want to override the faction's XML to change the starting units. For example, you could comment out the Initiate's build skill and commands, or just specific buildings from them, and then comment out some of the Summoner's skills and commands, etc.
