You can't know until and unless she does nothing but decline. That's failure to thrive. So definitely give her some Nutri drench or Chick Saver, keep her in with her flock with all that they all need anyway to be healthy and keep an eye on her. You'll know soon enough if she's going downhill or going to make it but be runtish or going to catch up like you say, since indeed that happens.But my question is, is she failure to thrive or what?
And i’ve also seen runts that end up being the biggest in the flock.