Yes, guides (or their companies) should be set-up as suppliers.

Supplier: the guide's name or company

Product(s): the services or tours offered

Option(s): how the services/tours are sold e.g. number of people, adult/child, half-day/full-day

If you want to add a tour, but you not sure what supplier/guide will run the service you can create a 'placeholder' guide - a non-specific supplier/product with general information about the tour. This can be added to a proposal and replaced when the supplier/guide is booked.