Hi Christy, welcome to the forum and so sorry to hear of your problem.
The best thing to do at this stage is to keep that QT tank water in excellent condition and watch for signs of infection. I would hesitate to add medications at this time. Plecos can be very sensitive to meds, and they have some incredible healing abilities if they are in pristine conditions and well fed.
Is your pleco still able to eat? This is going to be very important and be a big determining factor in whether he makes it or not. What kind of pleco is it? I'm also wondering about a 12 inch pleco in a 10 gallon tank... I can't imagine any way to keep that water quality clean enough to promote healing. Is it possible to move the pleco back to the larger tank and the small cichlids to the isolation tank instead? While stressing the pleco further isn't a real good idea, I don't see there is much choice. A 12 inch pleco isn't going to last long in a 10 gallon tank, even if its healthy. Check your water params on the larger tank, if there is any ammonia or nitrite, or nitrate is above 20, I would strongly suggest getting some Polyfilter into the filter asap. http://www.fosterandsmithaquatics.co...fm?pcatid=4335
Cut a piece big enough to fit into the filter with the cartridge, leave it in until it turns brown/green, but no longer than 4 days at a time. After 4 days, remove it, wait 1 day, then cut and place a fresh piece into the filter. This is a temporary thing, until the pleco starts to heal. Polyfilter can't stay in the filter full time for long periods because it will purify the water too much for the fish, but its the fastest and safest way to handle situations such as yours. Short term during recovery, it can be a life saver. I always keep this stuff on hand. Any of it that you don't use can be sealed in a zip lock baggie and stored in a cool dry place until a time comes you can use it. It has a shelf life that lasts yrs, so no worry that it will go bad anytime soon.
The polyfilter should help keep your larger tank in good enough condition for the pleco to heal if it isn't injured too severely and no internal organs were damaged. If you notice the skin around the injury turns pink or red, or begins to look fuzzy... post again and we will have to discuss medications to treat secondary infections. The polyfilter will help to avoid the risk of infection, so if you can get some soon enough, meds may not be needed.
Best of luck to you and your pleco. Let me know if there's anything more I can do to help.