Is the tank meant to be portable or stationary? Shiners don't care how cold the water is. It can be frozen as long as the air stones are functioning. I used an old bath tub for several years. You will need something that can withstand freezing(or use the heater) and hold at least 15 gallons of water. You could keep 5# in that amount of water at 55 degrees, if you wanted. You didn't say what you are treating them with. I would recommend Finer Shiner at the label rate. Get some non-iodized salt and use about a third of a cup when you change water. Mix the Finer Shiner and salt in a bucket and add it to the tank. That will insure that it is dissolved.
I forgot to answer your first question. No the tank does not need to be insulated.
Now, go fishin'.