We introduced a dummy really early at around 10 days to help settle DS after feeds. It hasn't had an impact on my supply at all. He still has his dummy (just to settle himself to sleep usually), and we are still b/f at 8.5 months. I had to work at increasing my supply in those early days as well, and I was wrried a dummy would cause more problems so I understand why you would be concerned.
Being a mum is hard work and like BG said, you have to go with what works best for you. It sounds to me like you are doing a fantastic job