A 10g should be fine for a small fish. If you quarantine all your fish before you place them in your DT, you really shouldn't have to quarantine them for parasites again. If you don't quarantine and then get a parasite, you may need a tank much larger than a 10g because you will have to place all of your fish into it.
3 weeks is the minimum time you should quarantine. 4 weeks should be fine if the fish shows no signs of parasites. If a parasite (Ick) is found any time during the quarantine process, then it would be at least 3 more weeks after starting treatment (hypo).