1) Most coldwater fish species will co-exist quite happily with each other. However, there are a few exceptions like Bullhead Catfish and species of Sunfish, but its unlikely you will be able to find these or even choose them for you pond.
2) According to your measurements your pond will hold roughly 1320 UK gallons which is fine for keeping Koi, the general rule is 1000 gallons, but remeber they produce alot of waste so stocking levels will need to be slightly lighter if Koi are included. But remember the water quality and filtration will have to be of a higher grade for Koi keeping.
3) From you diagram the pond will be a rectangular shape? If so all the corners will be of at least 90 degress so there will be no problem with the fish getting trapped. If you are using a liner I have always found it best to place the pond liner in the hole and fill it with water, it will then fill out the shape itself. Personally I prefer a more natural looking pond with shallow areas and un-uniformed edges, this also benefits fish in spawning and pond plantsing, especailly for marginal plants. If you want to attract wildlife the shallow areas will be a great help aswell.