how do you keep a quaranteen tank cycled

I think a few larger snails (mystery, apple) or many small ones (malaysian trumpets, common pond) would do the job, and clean the tank. Also, many fish diseases aren't readily transmissable to snails, making them perfect for a quarantine situation.

Any time you add fish to a mostly empty tank the de-nitrifying bacteria will need to catch up and this causes a mini-cycle. Frequent water changes during the first week the quarantined fish is added will mitigate this.

Mine doubles as a temp home for feeders (mostly glass shrimp), it ussually has 10-15 in there. It's ussually pretty stable, but I'm running an Aquaclear 20 filter so if I get ammonia problems during a qt I can always shove a bag of zeolite in there.

Another option is to keep a spong fliter going in a tank that has a lot of waste production and move it into the q tank when you are going to be using it. That is, if you want a spong filter in your other tanks.