Vivi's been gone long enough that her power could literally be anything. We have no idea what she did in those 2 years, maybe her and Kohza trained with each other every day or something. If Vivi needs to be strong enough to deal with actual threats then she will be.
As for Carrot we'll see in the next chapter whether it happens or not. I would like it to as I would like an ew crew member, but we'll see. One thought I did have though is that Carrot does have another reason to join now. She's been asked to be King but she clearly seemed hesitant about it, although not totally against it. The idea that people had for Momo - going out to sea to become stronger and learn how to be a true leader - could be used for Carrot instead. And with one of the themes of the next saga absolutely being what it means to be a King (Pirate King, Im, Vivi, Revolutionaries) this could track. But next chapter will decide that.