You kind of lost me with the tank on top of the well casing thing.
The important questions haven't been answered yet. What is your static water level?
Is this 100' your referring to horizontal/vertical or a little of both? And how much of each?
If you plan on putting an ejector in this six inch well, it will have to be a Four Inch Two Pipe Jet which will not do 15 gpm no matter what you do. Seven may be closer to the truth and the static water level will have a lot to do with that number. You will need two pipes down the well, 1-1/4" and 1", you will have to install an automatic regulator on the output side of this gas operated centrifugal to allow you to tee water back down to the jet to make it operate. The automatic regulator can be adjusted once installed to get the jet to work. This has never been done to my knowledge, but is feasable.