Do I need to work for more than one client to be a micro-entrepreneur?

At the moment I work 9.5 hours per week as a teacher for an association and I am paid a salary.

1. I also work for two language schools as a micro-entrepreneur. I bill them for my services.  Both language schools keep asking if I have other clients as they are worried about my status.  My feeling is that it is none of their business who else I work for.  Surely it is how they are employing people and on what terms which may cause them trouble and not whether or not I have other clients? Do I have to confirm, in writing, in French that I have other clients?
2. If I stop working for one of these language schools and do not teach any private students, can I work for only one language school as a micro-entrepreneur - or does working for only one client somehow put the micro-entrepreneur status in danger?  I would still be working part-time for the Association on a salary.
3. In addition, one of the language schools is asking me to confirm, in writing and in French, that “vous êtes à jour avec vos cotisations Urssaf” (which I am, and which I have confirmed previously).  This is the third time they have asked in 3 months and I am starting to find it insulting.  No other company has asked this.  Can I tell them to mind their own business and I’ll mind mine, or do they have a right to know?

Thank you for your help.