You cannot make "space" in the colon by taking probiotics. It does not work that way. kefir is no better at colonizing than any other probiotic, it is more likely to be worse.
Most bacteria in the colon protect themselves with biofilms and not much can get through to displace them. Bacteria, yeasts, fungi, viruses and possibly parasites go through the colon continually and very rarely have any effect on the colon flora. If they get a chance then they will take up an opening but that rarely arises. Even anti biotics don't usually affect the colon flora because biofilms are mostly resistant to them.
The colon flora is very static and rarely changes. However removing food sources could cause the colon flora to change which are the ideas behind GAPS and SCD. However, it's a slow process.
Oh BTW leaky gut is a very deceptive idea because it's not usually the gut that is the primary location for the increased permeability.
You are never likely to solve Increased Intestinal Permeability Syndrome (IIPS) aka leaky gut with probiotics! Biofilm busting procedures is the only thing which is likely to solve it.
The GAPS diet does have components to deal with biofilms but it is not very targeted IMO. However many of the components are very healing but it could do with more help.