I've never tried kefir. I'm not opposed to fizzy milk (I like colorado bulldogs, after all, and ice cream floats – I used to work at a university with proper dairy bar, good god I was so fat) so I might. Kombucha is fine. I'm pretty sure they bung half a tonne of sugar into many of these, so they're basically a standard soft drink with a pretend leanings towards being healthy.
We all love fermented stuff (bread, beer, kimchi, etc.)
Gut microbiomes are important, but I don't think you can specifically feed them, other than via a faecal transplant (there's fairly sound body of work on banking your poo, so you can reboot your gut flora after any course of antibiotics, though be wary when reaching into your freezer...) Even then, the stuff that lives in your colon is going to be very different to what lives in your small intestine, your duodenum, etc.
The gut microbiome is undoubtedly important (perhaps some of the claims are over-inflated, but I think it's vital for creating an effective, and not over-reacting, immune system), but it should look after itself if you eat the usual varied and healthy diet, and apparently it can shift quite quickly. That's why eating something new can give you the seismic farts, but after a couple of times, you're back in gentle tooting territory, but there's no way I'm eating Jerusalem artichoke again to test this theory.
Anyway the microbes in these probiotic drinks are very different to the ones in your gut anyway, so even if they got to where they need to be, they'd be bullied into nonexistence by the locals. It's only recently we've started to get an idea that lives down there, metagenomics is a product of fourth gen sequencers* and computational grunt. Anyway, it's a complex ecosystem of bacteria, archaea, protists, yeasts, and viruses, and probably some bigger parasites. Before metagenomics people had to try and identify them by growing them in the lab, but the majority of them can't be grown outside the gut. There are more microbial cells in our gut than there are human cells in our body.
*back when I sequenced DNA, I had to climb a ladder, now you have these. I am quite tempted to buy one just because.