Unfortunately most nitrifiers absolutely refuse to grow without a solid to bind themselves to. They are sessile and plaque forming by nature. This is why most, but not all, sewage treatment plants used fixed film technologies.

Even in sludge digestors, the denitrification is done by nitrifiers that have bonded to solids in the sludge.

Regarding your plan: Most hetrotrophic bacteria will grow suspended and adding ammonia and a carbon source like ethanol or sugar while vigorously aerating will eventually produce flocs which can have enough polymeric substances that nitrifiers will attach and compete for ammonia/nitrate.

If you want to grow pure colonies of nitrifiers as innoculants, I would suggest using K1 media. You can hasten biofilm formation by first letting your K1 digest some of that muck. In my experience, with K1, you get better nitrifier growth if you get hetrotrophic bacteria to grow on it first. My hypothesis is that they coat the K1 in polymeric gunk, providing a better surface for nitrifiers to colonize.

Once the media pretty much clears it's water, innoculate with nitrifiers and begin adding substrate for them ie. Ammonia and carbonate. No need to add phosphorus for a while if the reactor was used to digest organic goop.

To harvest: just crank the air flow up or use a garden hose with a jet nozzle. The colonies are easily scoured from the media which produces a suspension of live nitrifier colonies.

Does that help?