I always try and use conservative methods first. Epsom for bloat, swelling, constipation. AQ salt for external wounds, fin rot. The StressGuard I use for either of those conditions because it has an analgesic.
If conservative methods don't work, I would use the Kanaplex. Or, if the situation seems more serious, such as fin rot getting close to the body. Septicemia would immediately call for it, also popeye since it is most commonly an indication of a more serious infection. If there was bloat and epsom hasn't helped it, and the fish has been pooping, I would use Kanaplex.
Unless the cause of the bloat is suspected parasites- the poop being white and stringy, or parasites showing from the vent would be an indication. External parasites- if I saw the fish flashing and rubbing against objects, I would suspect parasites and use an anti-parasitic.
Good questions and excellent info here. I'm keeping a list of this stuff so I can be prepared for anything when I start my female sorority tank. I already purchased a small quarantine tank. (Hopefully it isn't TOO small. It's pretty tiny...but I only intend on using it when a fish is sick, not to keep in there for long. But I'm thinking 75% to 100% water changes could be needed, which could cause too much stress.) Hmm. I just want to do things right.