I don't know if you're still interested in information on the Lindsay photographers, but here's what I have. I did a little research a number of years ago, as I was also interested in dating old photos belonging to may family.
The oldest picture I have seen from Lindsay is a daguerrotype of James and Irvine Junkin, which from the ages of the subjects must have been taken in the mid-1850's. Most of the pictures of the various local families that I have seen from the 1860's seem to have been taken in Peterborough studios like R. Thomson and "Sun". The earliest photographer to work out of Lindsay for any meaningful period of time, as far as I know, would have been Thomas F. Gourley, who set up shop on William St. around 1864. Those of his pictures that I have seen are all cartes de visite (roughl 2 1/2" by 3 1/2") and incorporate a backdrop with a terrace and ornamtental wrought iron fence in front of a landscape.
By 1869 Francis Edward was working at Kent and William streets; I have never seen any identifiable products of his studio. G. W. Searle had also established a studio across from the old Post Office by this time; his pictures generally show their subjects seated at a small table in front of a backdrop of (painted) bookshelves. Searle appears to have been gone by 1877, while Gourley had moved to the corner of Ridout and Lindsay Streets. Hugh Johnston set up in the Doheny Block, but I have never seen any identifiable pieces of his work. James A. Scott and Mortimer I. Andrus were also in business by this time; as they were in business for some years, I believe that their pictures are the ones I have seen with a dark backdrop featuring a pillar and trees. Andrus left the partnership by 1884, but was still listed as a photographer in the 1901 census; Scott was apparently gone by 1886. All of these photographers seem to have produced cartes de visite only during the 1880's.
By 1880, Gourley had given up his business to be replaced at his "old stand" by Williamson and Hamill; this business was soon under the sole proprietorship of Eli Williamson. He was still listed as a photographer in the 1901 Census. The other two main photographers for this period were initially in partnership. Hamilton Fowler and Isaac H. Oliver had set up a studio by 1884, but by 1887 had started their own studios, Fowler with the Sunbeam Photo Gallery on Kent St., and Oliver on William St. opposite the then new Presbyterian Church. The pictures that say "Sunbeam Photo Gallery" on them would be from the earlier years of Fowler's Studio, as would the Oliver ones giving the location opposite the church. The earlier pictures would seem to have still been cartes de visite, but with more varied backdrops. The larger mounted pictures would soon become popular, however, certainly by the earliy 1890's. Oliver had moved by this time to premises at 128 Kent St. Both, however, were still recorded as photographers in the 1901 Census for Lindsay.
A couple of photographers worked in the town for short periods: George H. Wright around 1898 and William Thomas Pepper from before 1901, when he is listed as an artist in the census, to 1911, the year of his death. However, only one more photographer did a lot of work during the era of popular studio portraits. Maud Morton worked on Kent St. from 1899 until the early 1920's when a fire apparently destroyed her studio. By this time, the popularity of hand held cameras was making studio photography something to be utilized for special occasions only.
As to the negatives, very few seem to have survived; sometimes the odd cache of them will show up. I saw a few in the camera store in Lindsay three or four years ago, but I would say that unfortunately, the majority of these have not survived. The actual photographs were, however, produced for orders of as many as ten or a dozen, so the odds on these surviving are somewhat better; I have seen duplicates of a number of my family's photos in the hands of various more or less distant relatives, at family reunions and the like.
Another way of dating specific photographs is through the clothing styles, though it requires a fair bit of expertise. Otherwise, guessing the ages of the people in them (presuming you know when they were born) is still a fair guide.
I hope that this is of use to you.