I just crossposted into a community of a petition for something, which was removed with mod note "Please follow reddit's self-promotion guidelines.".
I also am a mod myself, mainly for r/Metrolinx. Just today I created a rule "content must be relevant; no spam", with an exception "Promoting subreddits related to transit in Ontario is permitted.". Am I allowed to allow such promotion of subreddits, or is there a Reddit rule that specifically restricts this?
https://preview.redd.it/h955sg1ru97g1.jpeg?width=1080&format=pjpg&auto=webp&s=12f6ec7cbced3aab33c8342edeee1e7fdb2a58ea
https://support.reddithelp.com/hc/en-us/articles/28012014962580-How-do-I-keep-spam-out-of-my-community
We like this guideline because we're not interested in any kind of self promotion from people who aren't active contributors to our community, but it's ultimately up to you.
Yes, you're allowed to permit cross-posted content that's relevant to your subreddit. Reddit won't have a problem with your sub for allowing that.
Reddit may have a problem with individual users who prolifically cross-post or post the same content across many subreddits, as that's spammy behaviour.
EDIT TO ADD: You can get a decent (but not perfect) understanding of reddit's rules on spamming here:
https://support.reddithelp.com/hc/en-us/articles/360043504051-Spam
As a user, you should avoid cross posting your subreddit to unrelated subs, or subs where you're not already active. If the only thing you post to a particular subreddit is your content or content from your subreddit, you may run afoul of their subreddit rules about self-promotion.