I don't believe there is such a thing as too deep. There is only "not deep enough"
From what I've read, fish spend 90% of their time in the top 4 feet of the water. That means that the rest of the water depth is rarely used. If your water level never droped, then four feet is enough. Since the water level will drop, you need a minimum depth of four feet based on the lowest level the water will drop due to drought and summer evaporation.
I know that's a crazy answer, but in my part of Texas, my ponds both drop about two feet at the most. So far. My big pond is 12 feet deep at the deep end, and 8 feet deep in for about half of it. There are areas that are just a few feet deep, and there are holes that are close to 20 feet deep. Fish like variety!!!
Somethint to keep in mind, is that plants will grow along your shoreline and even in the pond if the water is less th four feet deep. Some plants will grow down to 6 feet deep, but most like just a foor or two or water depth. Make your shoreline drop down as quickly as possible to cut down on how many plants ou get growning out into your pond. Shallow ponds can very quickly become overgrown. Deep ponds never have this problem.
How will your pond fill up with water? If it's from runoff after it rains, you might want to think about the silt that will come into the pond during those heavy rains. If you have really good, thick grass, then it wont be so bad. If you have dirt ,then you'll get silt into your pond.
I channelled ditches before my pond and only allow the water to get in there from three places. Two have culverts with pits in front of the culverts. After a heavy rain, those pits fill up solid with silt. If the pit wasn't there, that silt would end up in my pond, and the slow filling up process would be going that much faster. Any silt that I can keep out of my pond, the better off I'll be. At the third entry point for the runoff water, I dug an extra deep pit into the shoreline of the pond. I figure it's just a matter of time until it fills up, but at leat it will be going into that pit, and not into the bottom of my pond.
With silt pits, you have to dig them out when they fill up, or they stop doing anything. I have a backhoe to do this. If you don't have anything to do this with, it's still a good idea to have a silt pit. You just have to dig it bigger to catch as much as possible, for as long as possible.
The fish you chose to put in there will be more concerned with the size of the pond and what you have in it for structure. Stumps, longs, pallets and things like that are what they need to reproduce. Shallow nesting areas are also important. Depending on the size of the pond, you might be limited to two or three species of fish. One preditor, one sunfish, or food for the preditor fish, and feeder fish for the sunfish, like minnows. It takes several acres for a pond to support and grow multiple preditor fish. My small 3/4 acre pond has channel catfish in it for the preditor fish. My 4 acre pond has the catfish and large mouth bass in it, along with two types of sunfish and the minnows.
Hope this helps,
Eddie
We will have a pond built for fill dirt (doesn't hurt I wanted one anyway 
