Ehlers Danlos Syndrome (EDS) does create somewhat of a “loose spine” but you must be sure that you do not have other disorders independent of EDS. Mid-thoracic pain could also be derived from Scheuermann’s disorder (see website) or even a herniated disc.
I assume you have had standing X-rays and an MRI to rule in or out the above noted conditions.
The rubber mobilizer should be OK to use. This will help reduce the increased kyphosis that can be associated with EDS.
Shoulder subluxations are not uncommon with EDS. Strengthening the shoulder muscles can help.