Effects of logging in to same Apple ID on multiple macOS accounts (single computer) On my Mac running macOS Sierra, I have a "main user" that has no admin privileges. I also have a second user which has admin privileges. I invoke the privileges of that "admin user", when admin privileges are necessary for my "main" user.

I'm now considering logging in to iCloud with my standard Apple ID, with my "admin" user. The Apple ID is normally associated with my "main" user. What will be the system effects of connecting my admin user to that Apple ID? 

More specifically, I'm wondering if that will create stuff on my computer that will take up disk space for the admin user, such as iCloud Drive? Is there a risk of conflating application settings of the main user and the admin user which are stored in iCloud? And from a security standpoint, is it advisable not to login to Apple ID with my admin user (which I created just to isolate admin privileges from systems that shouldn't have them)?
If you turn on iCloud drive it will take up additional disk space, you can simply disable that part of iCloud to avoid that. Otherwise you will probably see a small amount of disk usage but nothing that will really affect your computer unless you're very low on available disk space.

As far as your other concerns I haven't actually tried this but I can't imagine you'd have any problems and if you were a client of mine I'd set it up for you. Think about how many people have 2 users or more users on their computer and share the same iCloud account. iCloud doesn't actually sync a lot of application settings and this would be no different than having two computers setup both with iCloud on them, which tons of people have without issue.