Partners get K1s, independent contractors get 1099s, but that doesn't mean the entity(ies)/employer(s) are going it right.
Who is (are) the employer(s), what is the tax structure, who are the owners, do they have earned income or compensation from the employer and who are the employees? If you have multiple employers, what is the relationship between them? Answer those questions and then you can determine the retirement plan options.
And if the income reporting does not properly match the corporate tax structure, you inform them thusly and if they have no interest in proper accounting you punt, not worth the headache IMHO.