Hello,
First let me just say you may want to double check that your dechlorinator actually neutralizes the fluoride, most, from my research, do not do this. Yours may have some different chelating agents that do, but I would double check with the manufacturer just to be sure. However, irregardless of weather or not it does I don't believe you have to worry.
I did some research on fluoride in tap water, essentially because you peaked my interest. This is what I have discovered.
The cheapest and fastest way to neutralize the toxic effects of fluoride is to add salt. I remember you mentioning that you have a high salt content in your water, this is probably why the water company is adding additional (more then the average) fluoride into your water. I think perhaps they are adding 3-4ppm of fluoride (I am guessing that is where you came up with that number) knowing that 90-95% of it is going to bind with the natural occurring salt making it inert. When salt and fluoride bond it creates calcium fluoride, which is non toxic to both humans and animals (at least per my Internet research). This is probably the other reason why they are adding so much fluoride to the water, to help neutralize the higher salt content.
So when the salt and fluoride combine you will essentially have calcium fluoride, some free fluoride (most likely within the national average of 0.2ppm completely fish safe per my research), some free salt (but less than is what is occurring in your water), and some free sodium. So the bottom line is you probably have much much less free fluoride in your water than you think, probably even less than the 1-1.5ppm they reported. I am not sure how the fluoride tests are done, weather or not the test differentiates between calcium fluoride and regular fluoride, so that might be why they are giving you the number of 1-1.5 fluoride - it probably does not differentiate between the two. So that should, if the information I have found is correct, eliminate your toxicity issues. Also, I found lots of people that have fluorinated water with no issues with toxicity. Also, as a side note, I discovered that naturally occurring fluoride can be found in almost all natural bodies of water ranging from 0.5-12ppm of fluoride, so again I don't think this will be an issue for your fish, but as you said it is probably species specific as to how much they can handle.
Finally, as for the fluoride causing problems with your bacteria colony, fluoride can kill certain bacteria, all information seemed to focus on bacteria that causes tooth decay, I don't think that is the same type found in our fish tanks. However, as mentioned before, more than likely, the majority of the fluoride in your water will be in the form of calcium fluoride, it is doubtful that this is the source of your problem as calcium fluoride is not toxic to aquatic life. Essentially the salt neutralizes the fluoride's toxic effects. Also as your neighbours are getting their water from the same source and have no problems it is unlikely that this is the cause of you stalled cycle.
Now as mentioned you will also have some free floating sodium in your tank, which I know will not be removed by dechlorinators/heavy metal removers as sodium is neither of these. I don't think the free sodium would have an effect, but that might be the next place we want to investigate.
I know this does not give you an answer of how to fix it, but I think we can safely eliminate the salt and fluoride as the cause. At least one step closer to hopefully finding the answer.