No modifications to the tennis balls. The intent is that everyone is using the exact same projectile. I recommend sticking with a good brand, such as Penn. If you look around ebay you can usually find a good deal. Cheaper brands are lower cost, but they just start falling apart.
How long of a break do you mean? Ultimately it's up to you. I think most guys just go right through without stopping (other than to tell the camera what round they're on). I do break up my own rounds when I record them. I (obviously) start my 13m round on the hour and throw the first 5. I then wait until 2 minutes after the hour (usually about a minute wait) to start the second 5. My 20m round is started at 10 minutes after the hour, and using the same two minute span, the second 5 balls at 12 minutes after the hour. I like having a short separation like this because it adds an aspect of live competition. At the World Cup in Ibiza, competitors with throw 5 balls, and then wait for everyone else to go before throwing the second 5. It can be a very long time between sets. I like including the separation even if it's only a minute break.
If you're thinking about short breaks like that, no problem at all. If you're thinking of longer breaks, just reach out to Channing and let him know what your plan is. If he has any concerns he can let you know.
Hope to see you in the OSC!