To start with, you should be looking at seperating the remediation company from the testing company. There is an inherent conflict with one compnay performing bith tasks (how do you ensure adequate remediation).
Most environmental engineering firms will have some experience with mold work. What you really need to determine first is what mold is present and why is it there. Without eliminating the source of moisture, any clean-up is doomed to failure as the mold will likely reoccur at some point in the future.
Mold sampling can either be performed by collecting bulk samples of the mold to determine species and family or by collecting airborne samples for the same info. Airborne samples will give you a quantity of airborne spores, which will allow yourself/consultant to compare post-cleanup results to ensure adequate remediation.
Unless you are attempting to save money, I would recommend that you hire a consultant to write a scope of work and tender the job based upon their sampling and investigation. They will be able to delineate the extent of microbial contamination and enforce procedures to ensure that the contractor does not spread the contamination. They should also perform clearance sampling afterwards prior to releasing the remediation contractor from obligation.
I would caution you from involving yourself with a moldy building. Many banks and insurance companies will hesitate to provide any coverage related to the mold. A medium-scale remediation can easily surpass $100,000. A small clean-up can cost anywhere from $5,000 to $50,000 depending on the extent of contamination and level of effort required. You are probably correct in that the cork is providing a source of nutrients for the mold to thrive on.
Another concern you may wish to investigate is whether the plaster contains small percentates of asbestos. It was often added as a binder to the plaster. Any disturbance to asbestos requires compliance with your provincial regulations which can lead to additional costs and liabilities.
Manitoba has an enforceable guide you should check out. Guidelines for the Investigation, Assessment & Remediation of Mould in Workplaces. Workplace Safety and Health Division, Manitoba Department of Labour. March 2001. Any work will be subject to complaince with this document.
I would recommend my own company, Jacques Whitford Limited, except Winnipeg is the only major Canadian city where we do not have an office. Make sure whoever you hire has experience with both mold assessment work and mold remediation work. Ensure that their insurance policies cover them for mold.