You can use the same key for multiple domains. Using different selectors for the same key won't help your reputation. It will also make signing more difficult. You can use the same selector for different keys as they will be published under different domains.
You will need to publish the selector DNS record for each sending domain. Sign the outgoing message with the matching private key.
In my experience, a high percentage of organizations either don't publish a DKIM record. Others publish an invalid record. Neither case adds credibility.
If you are sending from the same IP address, all domains will be marked as spam. Most blacklists are IP based, not domain based. As long as you are using a properly configured server, including DNS configuration it is unlikely your server will be flagged as spamming unless you send spam. Double opt-in should prevent that.
Do ensure your servers are not open relays. Open relays do get flagged and are commonly used to send spam. I do my best to ensure the spam gets left on the open relay as long as possible.