Regarding tank valves, we use the Apex valves from Tru-Test. They are a full flow valve that can put a lot of water in a trough in a hurry. We run 50 - 60 gpm coming off a 1 1/2" line. They'll do 20-30 gpm on a 1" line. They come in either high pressure or low pressure configuration. Low pressures work up to about 60 psi. If you have more pressure than that, they will not shut off. They will work down to about 4 psi so they do well on gravity systems also.
Our winter stock water source is a spring on the foot of the mountain. We use a continuous everflow/flow thru system to keep tanks open. At -25F we will get just a little bit of ice around the edge of the tank, but they never freeze over. Flow thru systems don't work so well if you have to pump water from a well as the well may not have enough capacity to run all the time, but they sure work well on spring developments (assuming it is a good spring!)