Using a bread premix - but wondering about the salt?

Hi Guys

My wife and I are both diabetic, so it's suggested that we eat multigrain or wholemeal. So, my starters have had mixtures of mostly all-purpose white flour and multi or german grain. I can get the sourdough ferment happening, but the bubbles are smaller and the batter doesn't float when tested? I wonder - if the combination of all-purpose and bread mix flour (this contains non-iodised salt) is slowing things? Cheers Steve