" Do these disposers ship with everything necessary to say replace another brand disposer that has failed? Are all the parts that hook them to the sink included?"
Yes, everything (except putty) is included to "hang" these disposers onto your current sink. As far as these disposers fitting up "exactly" to the same dimensions of your old disposer, that is nearly impossible to determine. Just like cars that change models and aren't always the exact same size/dimensions as the previous year, disposers models do vary somewhat from year to year and we can not "guarantee" that the new one will fit "exactly" in the same place as the old unit (even if it is the same model number and from the same manufacturer). Generally just a few parts at most are needed to make the change over.

"I've heard that garbage disposers waste harms septic tanks. Any truth to that?"
We have yet to read any study that reflects that. Some cities have banned disposers and then, later changed their minds and allowed them, such as New York. From what we know, food disposer waste gets consumed by bacteria in sewage treatment plants and septic tanks.

"My ___ brand garbage disposer smells! Is there anything that I can do besides buying a new one?"
Generally the simple solution to this is to clean the disposer by having it grind up a few cups of ice and some table salt. This helps to cut the grease and slime off the sides of the disposer. Then flush it out with cold water followed by half of a lemon or lime to deodorize. "Should I be adding enzymes/bacteria to my disposer or septic tank whenever I use my disposer?"
Not in our opinion. There are even disposers that add enzymes every time that you use that disposer. From what we have read, bacteria grow naturally after initially being introduced (mostly by nature) into septic tanks, and no additional enzymes/bacteria need to be introduced. From our perspective, adding additional enzymes/bacteria each time seems like a waste of money and resources. One additional note, our Founder/Leader wishes to encourage you to use composting and not throwing all wastes down your food disposers. Save the Planet and recycle as much as possible.