Do pre-built Little Snitch configurations exist and are they shared?
Yes, there are pre-built rule sets, and someone shares them.

I just stumbled upon this:

Little Snitch rules for blocking ad servers

Here they provide a list of pre-built rules for blocking ad-servers.

It is not related specifically to iCloud, but in general yes, there are shared sets of rules for Little Snitch.

Edit: now I see what Dylan was referring to with "are you going to manually...". A little explanation is in order.

At the time, the only way (I thought) was to copy the list and paste it into Little Snitch ruleset. It was not a difficult process (literally, cmd+c, cmd+v on Mac) but the "Rule Group Subscription" on LS makes it even easier.

The website I posted earlier now provides a link to the subscription: click and be happy.

I guess Dylan's answer and mine provide a couple of alternatives for subscriptions, but I wouldn't know the difference. I admit that I stumbled upon pgl.yoyo.org just by chance.

Further edit: It seems that Steven Black (linked in Dylan's answer) collects a few sources, including yoyo, so I'd say it is more comprehensive (more rules is better?).