I use "contaminant" in the sense of "accidentally imported by soil manufacturer, might be nonnative species"; I know they're harmless as long as they don't escape outdoors and outcompete the native Collembola.

The Oxford Dictionary says :
"contaminant" = "a polluting or poisonous substance that makes something impure".

BTW, the term used to express "accidentally imported by soil manufacturer, might be nonnative species" = "introduced".

(But one could certainly argue the collembolans, being a possible biohazard to Californian ecosystems and possibly also violating soil sterility laws, are nevertheless contaminants. But let's not get too itchy about linguistics.)

...I have also observed pieces of glass and plastic and highly invasive + horticulturally damaging angiosperm/hemipteran taxa in commercial potting soils, implying that the overall biosecurity of such companies is very poor. There was an intact zip tie in the bag I found these poduromorphs in.

"collembolans" is a bad anglification of the latin name "Collembola". Use simply "Collembola". Or "springtails".
You may use "collembolan" as an adjective, never as a noun.
E.g. : as a title of your new book : "The collembolan contaminant", by C.
;-)

I was not aware of the anglification thing until now since "collembolans" is a common word in many research papers.

But here you can see in a graph that "collembolans" is less common than "Collembola":
https://books.google.com/ngrams/graph?content=Collembola%2Ccollembolans&year_start=1800&year_end=2019&corpus=en-2019&smoothing=3