It is best to have little or no real rock (and no LR) and little or no sand. The reason for this is because these porous mediums will extract medications that you may need to use in the QT in the future. LR can not be used because it is alive with many micro organisms. Subjecting LR to QT treatments will be fatal to these creatures and the die-off that will result will pollute your QT. If you want to use some LR to keep your QT cycled while it is dormant and without fish, you can, but then I would not replace that rock in to your display. It can be easily taken care of in a cheap rubbermaid with a pump, feeding the rock every few days with fish food to keep it "live". If you do decide to keep LR in the QT during the dormant stage, then remember to do a major water change on the QT if medications were used.
It is best to not QT more than one fish at a time. The reason is that QT is more than just for treating an obvious sick fish. It is also a place for fish that may already be sick when you get them (such as the fish that you introduced in your tank that had ich) and fish that are stressed a calm resting place to recoup from illness and stress. In other words, finally get healthy for their final home in your disease-free display tank. Also, your QT will be a stripped down tank. Sources for natural filtrations, such as live rock and sand, will not be present. So, you seriously need to keep your bio-load down to avoid ammonia and nitritie spikes.
HOB is perfectly fine as long as carbon is not used when and if you need to use medications. Like live sand and rock, carbon will absorb medications which will compromises dosing your QT to meet therapeutic needs. However, after you have finished QT-ing a fish and QT is empty, it is a good idea to use carbon filters, and change frequently, to flush out the medications.
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