Paid relays are almost always pay to publish, but you can still connect to them without that privilege.

Clients don't care what relays you use, and for the most part will carry over to other clients if you publish them (this can be done on Iris.to manually, or apps like Damus do it automatically.)

I am still highly skeptical of the paid relay concept. I certainly understand why it exists, but I think it has a long ways to go. Not to mention, relays and managing relays is not an easy thing to understand. Even with quite a bit of study, I am still not 100% certain how to guide someone on how to best use or select their relays.