The moment you start opening up BP options, you are actively encouraging the use of this CAP outside of this core, and that is an issue. I don't care if come playtest people don't want to use this CAP within the core we set out for it to work with, but as a movepool leader, I sure as hell don't intend to encourage moves that support its use outside the core, nor do I intend to make it easier for people to ignore the core we set out to create this CAP for. If we give this CAP a bunch of moves that support BP teams, so much so that it becomes a BP staple, then I would consider this CAP a failure as per our concept.
I'm sorry, but this is not going to happen. Why would anyone in their own right run a SR weak Pokemon on a baton pass team to pass flame charges.
Additionally, saying we can pass the boost to another sweeper is also a moot point. If we did our job correctly in previous stages, CAP18 should have more synergy with Lucario than with other sweepers, so while other sweepers may enjoy the boost, lucario would be CAP18 favorite recipient. If flame charge raises usage of other sweepers, we have already failed.
If you think flame charge will make cap 18 into a sweeper, when was the last time you've seen Charizard Y run it, who has even more wall breaking power than us (and probably coverage considering how much we restricted that.)
While I do agree on avoiding throwing too much support moves around, flame charge isn't OP at all and probably won't see much usage. And if it does, guess what? We'd of learned that it's viable on whole lot of other Pokemon who haven't used it before.
On other moves:
Disallow Yawn. We switch in on Aegislash, they switch out to Goodra who gets hit by yawn. Proceed to either destroy the non-counter switch in or cripple your counter, not good.
Allow Perish Song, Whirlwind, and Roar. They phase, but they can't do what I stated above, so I don't have any qualms with them.
On a side note, we shouldn't disallow moves because they're unnecessary to our concept. Let the movepool submitters deal with things like that, becuase right now, they're all gonna end up the same besides flavor and a couple of coverage options.