Would a 10 gallon be too small for a quarantine

[GreenSpottedPuffer]

You should be fine if you do daily water changes as said above. You may want to give him a small piece of PVC to hide in, which is much better than a QT with rocks. If you need to medicate, you can leave the PVC in the tank but may need to remove the rocks and that will stress the fish. But if your QT rocks are always going to be only used for the QT then no worries...I am assuming we are talking about LR.