A 29g quarantine is a temporary solution even for 3 weeks at the very least. It should be flexible enough for any fish no bigger than 10 inches. Given the time duration to isolate, you'll have enough time to observe any unusual symptoms of the new fish.
The occasional scratching is nothing to be concerned of compared to frequent which is quite abnormal already as it delivers a message something is indeed wrong with your community tank. It's difficult to determine right now what is causing the scratching unless they keep rubbing the same body area in which case, they may either have skin or gill flukes. For ich, a few spots in various areas is enough for me to tell what is happening. Using the wrong treatment hastily can worsen your situation so keep a close watch for now and keep the treatments on hand. Neither of these scenarios can be treated by Pimafix and Melafix.
