This is an interesting approach. There have been at least 9 candidates on stage every debate (except Fox Business last debate with 8), so 3 candidates are potentially getting the boot here. Preempting this and taking a lead position to stand firm against the purposeful shrinkage of the debate stage will give Fiorina, Bush, or Kasich a reason to join Rand in refusing the undercard debate and not appear "whiny" doing so.
In fact, I might argue it even forces the other 2 candidates to have to follow suit, because then its a tacit acknowledgement their campaigns are second-tier by participating and submitting to the media. All Paul would need is 1 of them to join him and the other will likely have to follow. Once he's not alone in doing so, whatever narrative might have developed would immediately be flipped on its head and Paul might create a scenario that he helps keep the stage full at 9.
That would be some good press and maybe build a small surge of 2nd choice voters toward Rand from those who support candidates who might have been downgraded to the undercard.
I like it, even though I feel the undercard wouldn't be terrible.