Hi, received a PM asking me to respond. I have been traveling and haven't been on BTG for several weeks. Sorry I haven't replied yet.
Both idioms are allowed in general usage, but I'm not sure what the official GMAT stance is. I'm looking into it.
I will mention that, just because something is used in CR or RC does NOT mean that it would be okay for SC. Annoyingly, we've found multiple instances of idioms that were called "incorrect" in an SC explanation nevertheless used in RC and CR passages.

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Stacey Koprince
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Manhattan GMAT
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