Effects of logging in to same Apple ID on multiple macOS accounts (single computer)
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.