Azure B2B Multiple Office Tenancies For an Organization

If you already have an Office tenant, why do you want another one? they will be totally disconnected and you will have to do duplicate admin tasks. It would be much easier to just enable external sharing on dedicated sharepoint sites in your existing Office 365 tenant. 

You state that users of the new tenancy already exist in separate Azure AD directories. How can you know that for all of your customers? are these directories that you manage or that your customers manage?

B2B is good if you know who you are going to work with and want to preload a bunch of accounts. 

This should be helpful https://support.office.com/en-us/article/share-your-office-365-sites-with-external-users-89502322-bfbb-43d6-9207-4030f8ce26e0

In general B2B Guests can do whatever they are authorized to do by the person who invites them,with some restrictions of course. In SharePoint, they can be granted edit or read permissions

This may be helpful https://docs.microsoft.com/en-us/azure/active-directory/active-directory-b2b-what-is-azure-ad-b2b