My interpretation of tri-racial isolates is that they were light-skinned, culturally-white people who were marginally accepted in some communities in the Southeast when society was divided between free and slave. Whites further isolated them from their communities as the Civil War approached, and they were legally isolated after the Civil War when schools, churches, etc. were divided between white and black. The tri-racial communities refused to attend schools, churches, etc. with the newly-freed slaves and so formed their own separate communities.
Paul