If I currently am using the 5 free collaborators on the free plan and need to add a 6th collaborator how much will I be charged on the pro plan? (1 seat, with the 5 free collaborators still free or 6 seats for each of the 6 collaborators? )
If you switch a workspace to a Pro plan, that workspace will no longer be free, and you will need to pay for all 6 collaborators.
You could switch to a Plus plan instead of a Pro plan. You would still need to pay for all 6 collaborators, but Plus is less expensive than Pro.
If you do not need to be able to use an app, you could also see if someone has a legacy free workspace that they could transfer to you. A legacy free base can have an unlimited number of creators/editors.