I'll share a failure of mine from a while back that might be illuminating.
I once sterilized a bunch of saltwater + ground up fish flake food sample tubes, and then cultured up what grew from adding a tiny inoculation from roughly a half dozen different bottled bacterial products (heterotroph "grunge eaters").
Everything looked good - my sterile controls (that I just added distilled water to) grew nothing, and all the products grew nice cloudy solutions over a few days to a week. This result was repeatable, all looked reliable.
I then put all the cultured-up samples together to send to aquabiomics and had them tested, a survey of what strains from these bottles can be grown on common food inputs they might encounter.
The result that I got back was that by far the most numerous strain was something no manufacturer would intentionally put in their product - a fish pathogen (for some types of fish).
The lesson for me is that either bacterial manufacturers don't all have tight product control of what's in the bottles OR (more likely) the sterile technique required to reliably culture up a product is way beyond what a hobbyist like me is likely to achieve. Either answer means that culturing up bottled (heterotroph, "grunge eater") products to put in a tank would probably be a bad idea.
That said: culturing up the chemoautotrophic nitrifiers like in One and Only is much easier. It's what we do in every dark fishless cycle.
If you deprive the sample of organics and light, feeding ammonia and ensuring there's a little PO4 and plenty of O2, then the only things that can grow and multiply are bacteria that can consume ammonia (and nitrite) for energy - by definition the chemoautotroph nitrifiers you are looking for. Contamination is not much of a concern, like I said - it's what we do in every fishless cycle without light.
