I just revisited this and it seems like Rspec and Minitest work fine together "out of the box" without needing minitest-rails.
I'm not however using minitest/spec so I don't know about how that would integrate.
My issue was that the project in question was explicitly setting up individual railties in config/application.rb just to exclude "rails/test_unit/railtie" which is great if you just want rspec.
I put it back to the default
require 'rails/all'
and now both rspec specs run with
rake spec
and minitest tests run with
rake test
I wanted both to run buy default with just
rake
So I put this in my Rakefile
Rake::Task["default"].clear if Rake::Task.task_defined?("default")
task :default do
puts "Starting specs"
system('bundle exec rake spec')
puts "Starting Minitest tests"
system('bundle exec rake test')
end