James,
I am going to go in reverse order.
http://www.sea-temperature.com/ is a great site to determine the habitat of your shark. Keep in mind that these are surface temps and some sharks, depending on location will live at different depths. For instance the Zebra horn shark has a wide range from Japan to Australia. In Japan, it can be found in the shallows. Off Northern Australia it is found at depths of 50 meters plus. So yes, depending on the species you may be able to find an optimal temperature.
Sharks and rays usually do well together. Certain rays such as the Atlantic can become quite aggressive feeders and out compete the slower sharks for food. It is something to keep in mind.
Most ground sharks like the ones listed above usually do quite well together. Selecting fish to add to the shark tank takes significantly more time and consideration.
In my opinion, yes, ponds are preferable. They allow for more square footage and rounded corners. Stock tanks such as this one (
http://www.tractorsupply.com/america...lastic-2177188) are commonly used. The downside is that you now view the shark from the surface, which is suboptimal. Viewing panels can be incorporated into the pool but this does increase the chance of leakage and disaster.