The in-tank pump can only supply what it is rated at.
Absolutely, as does any and every pump. Remove the "load" or pressure restriction by putting an external pump downstream and now the in-tank pump is delivering fuel at near zero pressure (also called free flow). The in-tank flow rate during this condition is a maximum, it has almost no restriction at its outlet. So it is delivering fuel at 0 psi, or what is at "rated at", on its pressure-flow chart.
Putting a bigger pump down stream will only choke the bigger pump.
No way. In this configuration, the bigger downstream pump is now having fuel "handed" to it, instead of trying to suck it up itself. It now has an easier time delivering fuel (because it has fuel pushed to it) and will actually outperform it's "rated at" pressure-flow chart (which was developed with the single pump configuration).
It is impossible for a intank pump to flow more fuel just because there is a pump trying to pull fuel through it.
Incorrect. Ever blown in to a running fan, assisiting airflow? Have you noticed that it speeds up? Same principle. Again, the outlet restriction has been removed (the external pump is sucking fuel away) so the in-tank is in a free-flow configuration, delivering fuel to no load whatsoever. The bigger pump, pulling fuel away, is helping the in-tank pump, and in some cases, can actually cause negative suction pressures to develop and speed up the feeder pump. (This only happens when the external pump is free-flowing against no pressure load).
You will notice on pressure-flow charts, the slope of the curve at or near 0 pressure is negative (sloping down to the right). Almost always, the chart stops at zero presssure (since it would take another downstream pump to get data at negative pressures). But if you were to test with 2 pumps in-line, and develop negative suction pressures, you would see that the flow rate of the feeder pump would actually keep increasing (beyond that of free-flow) because it has fuel being pulled from it (like blowing through a fan). In this case, the p-f curve would continue upward to the left, with negative pressures on the x-axis. Seen this all the time while testing/charactrerizing multiple pump configurations.