No, I would definitely avoid adding plants until salt treatment is over.
Using multi cure, melafix and salt at the same time is not a good idea. Mixing too many treatments at once will usually do more harm than good. Also keep in mind afer 11 days the fish is suffering organ damage from the salt. I never keep salt in a tank for long than that unless I absolutely have to. A stronger treatment should be used once salt and melafix have been ineffective, so they should no longer be in the tank.
Saltypuppy - This is what I would do if these were my fish: If you've done a few pwc's since your nitrates tested high a short time ago, they should be in control by now. As long as the bettas aren't getting worse, I would continue to do two 50% water changes a week for the next month to keep the water prestine. If at the end of one month in perfect conditions, they show no re-growth, and you still feel certain they aren't nipping at themselves, I would begin a melafix and pimafix combo treatment. If that doesn't work, it's probably time to get a perscription for tetracycline.
