Cant tell from tiny picture but a sore mouth/red and sore looking generally indicates a bacterial infection. If the fish have white spot, carefully increase the temperature by a degree or two a day until tank is about 27-28 degrees Celsius and follow the indications on the white spot medication. (generally you will need to remove carbon from filters and treat every few days for a week or two, so long as a good 30% water change is done after the course, you can opt to repeat the course if whitespot hasn't cleared up, do another water change once treatment is finished and gradually decrease the temperature back down to about 25 degrees celsius).
Once the white spot has cleared up, replace the carbon back in the tank for a few days just to remove any residual medication. If the rainbow still has a problem with his mouth (you might find the whitespot medication helps it depending on the whitespot treatment) then I would then repeat the above directions but with a treatment for bacterial infections and also without turning up the heat as that isn't necessary for bacterial infections.
Personally I would be using Waterlife Protozin for whitespot and Waterlife Myxazin for bacterial infection.
Alternatively you can go all out with two ESHA products, ESHA Exit and ESHA 2000 (I think that's what they are called, one in red box and the other purple), they are designed to be used together to target a wide range of problems, exit namely treating whitespot and 2000 namely treating fungal and bacterial problems.
So long as you don't exceed the dosage instructions, all of your species should be fine, they aren't the strongest medications and aren't known for killing off delicate species, however if it does make you nervous, I go by the rule of 'half the dosage, double the course', it wont be as effective as going by instructions but better than shocking the fish with the full dosage.
To be fair, the only fish I have found to be really delicate was the ghost glass catfish and they *definitely* don't like salt anywhere near them. All the other fish like loaches and knife fish that are supposedly more delicate seem to be more tolerant than they are sometimes made out to be...