Any notes that are visible ONLY on relays Primal doesn't cache from would not be visible on Primal, regardless of what relays you select. In Primal, you only WRITE to the relays you select, and you only READ from their caching relay.

That said, the likelihood of any notes only being on a relay that Primal isn't caching from is relatively small. Most people are posting their notes to multiple relays with at least one that Primal is pulling notes in from.