Self employed via adr network - help needed

Hi I approached ADR for work and they have signed me up but have asked to register me through nova as self employed?

I am a bit weary of this, and have a couple questions I hope someone might be able to clarify for me?

Firstly what is the situation if I only end up working a couple days in the week with this agency would I still be able to work for another agency on a PAYE basis? And how would it effect what tax I pay to them

Also if work was to dry up and I required to sign on to help tide me over, as a self employed person would this still be possible?

Any help and advice grately appreciated