Consolidating communities could be done sort of like multireddits, basically just an organizational tool, which didn’t confuse people about the different rules, because it was clear which subreddit individual posts were on, and rules were clearly displayed in sidebars.

It could work in a different way, where the communities are more thoroughly merged, but there would be no reason for it to be anything but voluntary for the moderators of the individual communities. Presumably, the rules of the combined community would be something that would be discussed and decided on between the moderators of the individual communities that would enforce the rules.