6MWT have been used as a performance-based measure of functional exercise capacity in many populations including healthy older adults. I have never seen any study using 6MWT to assess tolerance to exercise with your population, yet, I believe 6MWT would be an excellent choice. You just need to justify why you are using and that it is an innovation. You could also explain that it is fairly safe, therefore a better choice. Good luck!
I'm using the 6MWT as a measure of functional capacity in people with stroke who are participating in a research study for a rehabilitation program. I think it's a very useful tool