Changing cleaning contractors is not a clean break. Ontario's Employment Standards Act has a specific set of rules for building services providers, which covers cleaning, security and food services at a premises. When the contract for a building changes hands, the incoming provider must in most cases apply the ESA's termination and severance rules to the outgoing provider's employees, counting their full length of service with the previous contractor rather than starting the clock over. There are narrow exceptions, including where an employee did not work primarily at that premises, or turns down a reasonable offer of employment. It is worth reading the rules before you tender, because the cheapest bid can carry a termination liability that the price does not show.
Ask for the WSIB clearance, every time. A clearance is a number the WSIB issues to a registered business showing it is signed up and current on premium payments and reporting. Only a registered business in good standing can get one, it is free to request, and it stays valid for up to 90 days. Asking for it takes one email and tells you something a glossy proposal cannot: that the crew working in your building overnight is actually covered.
Between those two, most of the risk in a commercial cleaning contract is administrative rather than operational. The contractors that handle it well tend to publish their insurance limits, bonding and certifications openly, which is why we weighted that so heavily above. In Toronto the practical wrinkle is that most large buildings are run by a property management firm rather than the tenant, so the cleaning contract, the ESA liability on a change of provider and the payment cycle all sit with a third party who did not sign your proposal.