I've never seen the term "not negligible" but perhaps the small print is so small that I can't read it.
A standard policy for simple four seaters would have all pilots as approved by the insured (hence check rides) with an excess of 1% of insured hull value with hirers liable for the excess. Adding individuals or other companies for cross hire as "insured" is no extra cost, just letting your broker know.
No need to take out your own insurance if this sort of cover is provided, you can only claim on one policy anyway.