Regular messages (that are not in Campaign) won't interfere with the ‘Rate limiting’ of your Campaigns. Users who received a regular or one-off message will still be eligible to receive Campaigns according to the selected ‘Rate limit’ defined in Campaigns. This means you can schedule and send regular messages to your users without worrying if it will delay your Campaigns from sending.
As an example, you might have a Campaign with a rate limit of "at least 1 day apart" and decide to send your users a one-off message. In this scenario, users who qualified for the Campaign on the same day as the one-off message would receive both those messages that day.