Of course, then it triggers all the bushes on the screen. You'd be better off just setting the undercombo. Unless, of course, you have other things on the screen. Then the flag might be the best option. But then you might want to use the "No secret sounds" screen flag too.
No, it doesnt. If a secret triggers on the screen, yes, then it will reveal all of the bushes that are assigned secret combos. But if you're just slashing them, they simply reveal the secret combo beneath them. Slashing a bush over a secret combo does not trigger other secrets.