Just use a fresh one, not one that has held plants before. If you really have an urge to clean (this new terra cotta pot) a good rock/kosher/aquarium salt bath won't do it any harm, and might soften out some sharp edges.
Just don't add any soap!!
I still highly recommend covering the hole somehow, whether with substrate (gravel) or another safe, non sharp method. Those fish get curious, swimming back and forth all day and night. One of them will try to see what is behind/below/above that hole, and there is a very good chance they will get stuck. It is not a pleasant feeling trying to get a stuck fish out of jagged hole like that. It is actually a little bit heart wrenching.
Other than that, I have been extremely satisfied with my own terra cotta plants, and I think they are a wonderful addition to any aquarium.
I recommend having them on their side, however, as right side up might not allow the fish any enjoyment or benefit of it.